Prosthetic f, backformation or eagal and feagal

Prosthetic sounds are nothing unusual in the… ah OK, right you are. A prosthetic sound is a "random" sound that gets stuck in front, the middle or at the end of a word to make it conform better with the sound rules of whichever language.

Many languages do this, such as Spanish. Spanish phonology has a rule saying that no word may begin with [sp] [st] or [sk]. However, there are words in Spanish which either have these initials historically or have them from loan words coming into Spanish. To get around this problem, Spanish phonology says "prefix [e]" … so Latin spīritus, scola, stabilis become espirito, escuela, and estable. Similarly Scandinavia, spot and Stockholm become Escandinavia, espot, and Estocolmo.

Errr and backformation?

Backformation means that speakers of a language take a native or adopted word and re-analyse it, as it were. This often happens in loanwords or native words whose meaning has become opaque. Opaque, when a linguist uses the word, means that the meaning is not clear from just looking at the word. For example, you can look at the word drumstick and figure that it's a stick that's used with a drum. But no matter how hard you stare at a hot toddy, chances are you won't figure it comes from a Hindi word for a palm-tree.

Where was I? Ah, backformation. English, for example, adopted the French word for cherries, cerise from Latin ceresia, both mass nouns which means there is no singular form. But because English has a very prominent -s plural, English speakers figured that the s must obviously be a plural. So, if many of those little fruits are cherries, one must obviously be - a cherry. This is called backformation. Gaelic does that too, but because it has the phenomenon of lenition at the beginning of words, unlike English, it tends to create backformations at the beginning of words.

Now, Gaelic phonology does not forbid words beginning with a vowel (don't laugh, some languages have no words beginning with vowels!) but it does have something like prosthetic sounds. Most notably [f]. In the case of Gaelic this comes about when native speakers re-interpret words beginning with a vowel as "a word which has lenited X". However, there is more than just one backformation. Let's have a quick look at what can happen:

[h] ⇨ [t], [d] or [s]	because lenited [t] or [s] yields [h], failing that, you drop it. [d] is rare, possibly via [t].
[ð] ⇨ [d]	because lenited [d] used to yield [ð]
[θ] ⇨ [t] or [s]	because lenited [t] used to yield [θ] and presumably when that linguistic memory faded, [s] in the same way German speakers often pronounce it [s]
[v] ⇨ [u] or [b]	because lenited [b] yields [v] or failing that, sounds a bit like [u]
[j] ⇨ [g]	because lenited [g] yields [ʝ]. This happens in words with a soft onset which is an almost [j] sound, as in English iota, which to Gaelic ears sounds like the word begins with gh th or dh
initial vowel ⇨ [f]	because lenited [f] is silent - and this is by far the most common one, and about the only one, still active in modern Gaelic
initial vowel ⇨ [t], [s] or [ʃ]	Because the masculine article in front of a vowel is an t- and the t- is sometimes wrongly attached to the noun itself. In rare cases, via t » t-s » this might become [s] or [ʃ]
[sw] ⇨ [sb]	This is rare but works on the basis that [sw] is not a permissible initial in Gaelic but the phonetically close [sb] is, i.e. a simple de-lenition of [w] ⇨ [b]

There are more, but those are the most common ones.

Examples

Examples to your heart's content, most of them are loanwords, except the ones beginning with f + vowel from Old Irish. Note that in most cases it's almost impossible to tell if a loan was from Scots or English, so they're usually listed together unless it's very clearly a Scots word. English can also stand for Old or Middle English. It's not that relevant to this topic, so I'm taking a shortcut!

Source	Meaning	Gaelic	Irish	Manx
Brythonic linn	pool, lake (cf. Welsh llyn	Gleann Iucha Linlithgow with linn being re-interpreted as lenited and slenderised ghlinn
Latin ferculum	dish for serving food	searcal choice part of meat	searcal dish; tidbit
Latin fundus	foundation	bonn; fonn	bonn
Latin Ianuarius	January	Gèanair, Gion-bhair, Gineamhair	Eanáir
Latin jocosus	droll	geocach itinerant, vagrant (original meaning referred to a town crier/street performer)	geocach
Latin iūdaeus	Jew		Giúdach
Latin theōrica	theory	teòirig Old Irish teoiricecht	teoiric
Latin Thūlē or Greek Thȳlē	Thule	Tìle though this may already have had initial /t/ when borrowed from Latin. It depends largely on the exact pronunciation in the giver language at the time	Tuile
Latin vāgīna	vagina	faighean	faighin	fine
Latin vanus	empty	fànas	fánas gap	faanys gap
Latin vēlum	veil	fial	fial
Latin vesper	evening	feasgar	feascar vesper	fastyr
Latin (in) vetere lege	(in) old law	peitearlach
Latin vicus	village	fìch	fích
Latin vigilia	vigil	féill	féill	feaill
Latin vīnum	wine	fìon	fíon	feeyn
Latin vulpēs	fox (possibly whelp/hvelpr)	uilp
Norse eyrr	gravel bank	tiùrr
Norse haf	sea, ocean	tabh, also tabh-bhéist, seal; tabh-uan seal pup
Norse háfr	hand-net	tàbh/àbh	tábh, abhadh
Norse hár + bolstaðr	high + farm	Tàbost
Norse haugr	howe, mound	Togha Mòr/Beag, Toghsgaig
Norse hjálm	helm	failm/ailm
Norse hjalt + land	hilt + land	Sealtainn	Sealtainn
Norse hlíð	gap (in a fence)	(cachai)leith
Norse hnúkr	knoll	cnuachd
Norse hǫll	hall	talla	(halla)	(halley)
Norse holmr	holm, small island	tolm, An Talm Holme	tolmán
Norse hóp	small land-locked bay	tòb/òb1
Norse hvalr	whale	Bhàlasaigh, Valasay; Bhàlaigh, Vallay
Norse hvarf	turning	Am Parbh, An Carbh Cape Wrath
Norse jarl		iarla	iarla	eearley
Norse þeisti	black guillemot	taibhse
Norse þerna	tern	*teàrn » steàrn(ag)	*tearn » stearnal
Norse þili (or poss. OE deal)	wainscot	déile	tile
Norse þingvǫllr	assembly field There are even examples of delenition in Scots here: Dingwall, Tinwald, Tingwall	Cnoc an Tiongalairidh Tiongal, Eilean Thingeartsaigh Eilean Thinngartsaigh		Tinvaal
Norse þópta/þófta	thwart	tobhta	tochta, tafta	thowt, taft
Norse Þórkætill	Thor's cauldron (name)	Torcall
Norse Þórir	Thorir (name)	Tòrasdal, Torsa
Norse Þórmóðr	Thor's courage (name)	Tormod
Norse Þorolf	Thor's wolf (name) The English form Torulf is a rare case of backformation in English.		Torolbh
Norse þorskr	cod	trosg	trosc
Norse Þrasi	Thrasi (name)	Trosaraidh
Norse þræll	slave, serf	tràill, Tràilligil	tráill
Norse Þrondr	Thrond/Trond (name)	Tròndairnis, Tranntail
Norse Þrum	Thrum (name)	Truimisgearraidh
Norse þurs	giant, ogre	tursa (tursaichean/tursachan)
Norse þytr	whistling sound	sitrich	seitrighim
Norse svartbakr	black-backed gull	farspag/arspag; farspach
Norse vaðill	ford	fadhail
Norse vágr	bay	bàgh/-bhagh/-bhaigh		baie
Norse vík	bay, cove	ùig
Norse víkingr	viking		Ó hUiginn Higgins, uiging pirate fleet
Norse vindauga	window	uinneag/fuinneag	fuinneóg	uinnag
Norse Vörðufjall	ward-fell			Barrool
Norse wrek	wreck	frac/fracas/fracaid	raic
Old Irish abra	eyelid	fabhra/abhra	fabhra/abhra
Old Irish acallam	addressing, conversing	agallamh	agallamh	taggloo
Old Irish (f)acus	nearness	fagas	fogas	faggys
Old Irish adba	den, lair	sadhbhaidh
Old Irish aicill	preparing, readiness	faicill	faichill
Old Irish aicsiu	seeing	faic(s)inn	feicsin(t)	fakinagh
Old Irish áinne	ring	fàinne/àinne	fáinne	fainey
Old Irish airiugad	perceiving	faireachadh	aireachtáil
Old Irish all	cliff	faill/aill; mac-alla/mac-talla (cf. Norse fjall which itself got borrowed in place names as -bhal, for example Ceapabhal) and mac-alla in Gaelic, which then either via the old aspiration of -c- as /kʰ/ or backformation of -alla as -talla has yielded mac-talla)	aill/faill; macalla	mactullagh
Old Irish allas	sweat	fallas/allas	allas
Old Irish anad	staying, remaining	fanachd, fantainn etc	fanacht	tannaghtyn
Old Irish ásaim	I grow	fàs	fás	aase
Old Irish atúd	kindling	fadadh/adadh	fadú	foaddey
Old Irish ecla	fear	eagal/feagal	feagla/eagla	aggle
Old Irish ed	distance, interval	eadh/feadh	feadh	feayn-
Old Irish (f)íarfaigid	inquire	fiafraich/feòraich	fiafraigh
Old Irish filliud	folding	filleadh/pilleadh	filleadh	filley/pilley
Old Irish étáil	gain, wealth	eudail/feudail	éadáil
Old Irish fogal	pillaging	othail/fothail	fothal
Old Irish forud	shelf, bench	àradh/fàradh		aarey
Old Irish fuiseóg/uiseóg	lark	uiseag/fuiseag	fuiseog	ushag
Old Irish ilur	eagle	iolaire/fiolaire	iolar	urley
Old Irish nóinín	daisy	neòinean/eòinean/feòinean	nóinín	neaynin/eaynin
Old Irish omhtann	thistle	fothannan		onnane
Old Irish osclaicim	I open	fosgail	foshil	foshil
Old Irish rádharc	vision, view	radharc/fradharc	radharc	reayrt
Old Irish taidbse	ghost	taibhse/aibhse	taibhse
Old Irish uacht	cold (n.)	fuachd	fuacht	feayght
Old Irish úaig	sew	fuaigh	fuaigh	fuailley
Old Irish uar	cold (adj.)	fuar	fuar	feayr
Old Irish uath	hate	fuath/uath	fuath
Old Irish urusa	easy	furasta	furasta
French jeune	young	diùn-laoch young hero
Welsh Gwgan	gwg "frown"		Úgán "Wogan"
Scots hough		toch/spaoileadh
Scots swele	swaddle	spéileadh/spaoileadh
Scots swearing	swearing	speuradh
Scots webster	weaver	buabastair
Scots yair	fishing weir	geàrr	geárr
English advantage			buntáiste
English churn		ciùrn
English flat		blad-
English flatter			bladar (?) flatter
English haar		càrr
English hackle (?)		sisteal	siostal
English haggis		taigeis	(hagaois)	(haggish)
English haggle		taglainnich
English hail	goal	tadhal
English handsel		sainnseal
English Harold		Arailt	Arailt
English harlot		tàrlaid
English hasp		deasp/iosp/easpach
English hatch (n.)		saitse	haiste, aiste
English halberd		taileabart/aileabart	halbard
English hay (n.)		saoidh(e)
English heckle (n.)		seiceal	seiceal
English hoe		tobha
English hogshead		tocasaid	oigiséid
English hoist		soidhseadh
English hops		top/sop/op		ob
English hubbub			siobob
English jib		sioba/seibe	gib
English jot (poss. L. iota)2	a bit	tiota2	giota
English joust			giústáil
English osprey			fáspróg/ospróg
English palfrey		falaire/alaire	falaire
English thaft		tobhta	tochta
English thank		taing
English thole			dola thole-pin	tholley thole-pin
English thousand		sùstan "busy"
English thrang		trang "busy"	trang(láil)
English throat				troat
English throne		"busy"	trón	troyn
English tuck(er)	stretch cloth (i.e. fulling)	tùcadair/ùcadair/fùcadair	úcadair	tucker
English use/usage	(Possibly)3	ùsaigeadh/ùisneachadh	úsáid
English vassal		basaille	basáille
English vault		buailt
English venturing		meantraig
English verdict			béirdict
English vervain		bearbhain	beirbhéine
English vestry		beastraidh	beistri/beistéir
English vicar			biocáire
English vice	(tool)	bidhis	bís	baise
English victuals		biotailt/biteal(t)
English vigil			bigil, uigil
English village			biláiste
English vinegar		binigear/minigear	biniger
English viol		biol	bíol	biall
English viscount		biceas	bíocúnta
English voyage(r)		bàidse/bàidsire
English waggon		baighein	báigin
English walewort		mulabhar/mulart
English wall		balla	balla	boal
English waistcoat		beasta
English waistcoat		siosacot/siostacot	bástchóta
English ward		bàrdaig/bàrdainn warning	barda, bárdaim
English wardship		warning	bárdéis
English ware(s)		bathar, bàirig
English warn/warning		bàrnaigeadh, bàirligeadh
English warrants		barantas guarantee	barántas	barrant
English wear				baarail
English weevil		uaimheal
English well		buthail recess
English welt		balt	bálta	quelt
English whack			faic anything
English wheel		cuidheall	faoileáil (?)	queeyl
English wherry		curaidh
English whig		A' Chuigse Whigs	uig/fuig, An Fuig Whigs	quig
English Wight	Wight		Inis Iocht Isle of Wight, Muir nIcht The English Channel
English whinger	whinyard	cuinnsear
English whip		cuip	fuip	kip
English whisk		fusgan
English whisker			foisgear
English whisk(e)y			fuisce
English whisper (probably)			piosarnach
English whist			fuist	quiss
English whores		siùrsach (sing.)	siursach/siortach/piurtach (sing.)
English wick		buaic		bite (?)
English win		buinnig
English William		Uilleam	Uilliam	Illiam
English wish				bwooishagh
English witch		buidseach	buitseach	buitçh/buitçheraght
English woad			bód
English woodcock		fudagag/udacag/budagoc
English wormwood		furmailt/burmaid
English wormwood		mormanta	mormónta
English worsted		bursaid	mustairt
English wrack		bruig
English yacht		geat
English Yankee		Geangach	geál
English yawl		geòla
English yeast		giost/giosg	giosta	jiastyn/jastee
English y(e)oman		gìomanach	gíománach
English yoke		geòc yoke but also geòcaire etc (semantic shift from yoke to throat, neck)	geoc	jokal

¹ So basically some areas dropped the h- and some turned it to t-, which is also the explanation for those awful spellings with t-H or weird t- initials you sometimes see on Gaelic street signs like An t-Òb/Muinntir an t-Òb (Leverburgh on Harris for example). It's nothing of the sort, it's just a case of tòb i.e. the place is actually called An Tòb - which makes muinntir an Tòb a totally regular form without any need for grammatical contortions.
² Although tiota in Gaelic is conceivable a case of the t of the definite article latching onto the noun i.e. an t-iota > an tiota. But with Irish giota clearly a backformation, it could equally be iota > *thiota > tiota.
³ It depends on the timing - modern English has a /j/ at the start of use but the original loan from Norman French us did not have this initial /j/. So if it is a really old loan, the ù just represents English /u/. If it's a more recent loan, it's a backformation of /ju/ » /uː/.

What on eart?

On occasion, this also took place in the middle or at the end of a word, though much more rarely:

Source	Meaning	Gaelic	Irish	Manx
English broth		brot	brat	broit
English Cromwell		Crombail/Crombalach	Crombeoil
English privet		priobaid though it's not entirely clear if this was deliberately backformed

Of course in Hiberno English (the English spoken in Ireland), this particular change is so common it's practically the default i.e. wherever in the word English has a /θ/, you get dental /t̪/ and where English has /ð/ you get dental /d̪/, so not only does three become tree but also math becomes mat.

English	Hiberno-English
English cloth	*clot
English earth	*eart
English lethal	*letal
English math	*mat
English thick	*tick
English third	*tird
English this	*dis

I thought Gaelic had words with f and p or indeed s?

Yes, it does, but initial f seems to occupy a strange space, perhaps because it is the only initial that actually lenites to zero (linguist speak for "it lenites and leaves no sound trace") and because p itself lenites to /f/. As a result, there is a regular merry-go-round between words with f, p and initial vowels - and on occasion even m:

Source	Meaning	Gaelic	Irish
Latin fenestra	window		seinistir
Latin feretrum	bier	eileatrom, eileadram	eileatram
Latin fūstis		sùist	súiste
Latin marculus	small hammer	fairce/fairche	fairce/fairche/farcha
Latin parochia	parish	fairche	fairche
Norse floti	fleet	plod	plod
Norse hjálm	helm	palmair (via failm)	pailmaire/failmaire
Norse páll	hoe or spade	fàl	fál
Norse wrek	wreck	prac/pracas
English fencing			pionsóireacht
English ferret		peireid
English fine/fining		paighne / paighneachadh
English Flemish (language)			Pléimis
English floating		plodadh
English (the) flux			pluicis
English flour		plùr	plúr
English flower		plùirean	plúr/plúirín
English Frances			Proinseas
English fry		praighig	priáilte
English frying pan		praidheapan
English mint		pionnt
English pewter		fleòdar/pleòdar
English pheasant			piasún
English physician			pisicidhe
English powder		pùdar/fùdar	púdar/fúdar
Spanish patata/batata (poss. via a 3rd language)			fata

Add a pinch of madness...

Even without turning to loanwords, this can get downright crazy when you throw the effects of nasalization and rebracketing into the equation. The Old Irish root of the word nettle is nenaid which eventually yields neannt + ag. But this is where the fun starts. Because this is very common with the definite article, an neanntag quickly results in people thinking of this as an eanntag. Which some folk of course mis-interpret as a case of an fheanntag, giving rise to feanntag. Not to be outdone, some folks took an neanntag and considered it to be the result of nasalization i.e. they assumed the word had to be an deanntag. The only ones missing for a full set are *teanntag and *peanntag!

There are several other close contenders for the Top Morpher prize though:

Modern Gaelic	Meaning	Variants	Root	Source language	Notes
builmean	bubble (Scots bummel being a by-form of bubble)	puilmean, fuilmean, cuilmean	bummel	Scots/English (?)	The c- form is either backformed p » c or the whole root may be based on an undocumented *uilmean
deodha	hemlock	a-teodha, a-theodha, de-theodha, i-teodha	dedga	Old Irish	Historically this root seems to have referred to various plants, in particular the centaury, henbane and hemlock. All Scottish Gaelic incidents of this root collected in actual field-work (Lhuyd ettiô, DASG, Dieckhoff tetheotha) gloss it as hemlock. Dineen and O'Reilly gloss deodha as 'henbane', which may be the source of various entries in Scottish Gaelic dictionaries which gloss some of the forms of deodha as 'henbane'. Deriving deodha from dedga is fairly trivial, what's slightly more perplexing is the set of suffixes it seems to have picked up, even though between them they just represent confusion of d as an t- Old Irish glosses give dedga/deadhgha as meaning 'centaury' but note that this was an ambiguous term in older usage referring at least to two distinct plant families, centaurea or centaurium. Henbane is rare in Scotland, especially in Gaelic-speaking areas, so the name may have transferred from an earlier plant to the much more common hemlock.
de-tibheach	gullet	fe-tibheach, i-tibheach, le-tibheach, tibheach	díbechán, dimechán	Old Irish	the derivation of díbechán, dimechán itself is not clear but either way, the dí probably first led to re-analysis of the d as being an t-, resulting in i-tibheach, which then easily would have resulted in re-analysed fe-tibheach. The le- form is rather unusual. Suggestions?
fadag-chruaidh	weather-gall	adag-chruaidh, badag-chruaidh, fadadh-cruaidh, fadag-ruadh	?	?	These are clearly related but it's hard to say what the underlying form is. There is one form, ceanna-cruaidh which seems to suggest cruaidh as part of the root but it could equally be that's the underlying form here is something like *ceannag-rua(i)dh. Which might hint at these all having ruadh as the second element i.e. fadag as fada + ag ("stump"?) plus ruadh i.e. "red stump" (red/yellow being quite dominant in rainbows).
failm	tiller	ailm	hjálm "helm"	Norse	the derived form failmadair has a de-lenited form palmadair and the form talman ("rudder bar") is also almost certainly related.
falmaire	hake	talmadan and colmaire (including Irish colmóir)	kolmúli "coal snout"	Norse	The o «» a variation (cf fola/fala could easily yield *calmair(e) and from there *almair(e) and finally falmair(e). The form talmadan makes makes it look a tad shaky though. Compare also Danish kulmule
feachaireachd	horseplay, based on eachaire "equerry"	peachaireachd	ech "horse"	Old Irish	though the root seems fairly obvious, *eachaireachd is actually unattested
feaman	tail, stump (ultimately from the same root as feamainn)	eaman, seaman	femman "seaweed"	Old Irish	presumably via *teaman or directly via an t-eaman
feanntag	nettle	eanntag, deanntag, neanntag	nenaid "nettle"	Old Irish	oddly enough, the form closest to Old Irish, neanntag, is very rare today
feilmigear	ridgeling	seilmigear, teilmigear	?		and I haven't got a clue what the underlying word is for once!
maighdeag	a cowrie (lit. small maiden because, well, google a picture and use your imagination)	baighdeag, caighdeag, faighdeag, paighdeag	maigd- "maid-"	Old Irish	The caighdeag form is decidedly odd, though it's possibly a p » c backformation OR hyper-correction of nasalisation (though with is being a feminine noun, that makes this less likely)
muir-bhuachaille	diver (bird)	mura-bhuachaille, muna-bhuachaille, buna-bhuachaille, bura-bhuachaille, burbhailleach	muir "sea" + búachaill "cowherd/servant/boy/guardian"	Old Irish	A little conjectural but muir- seems to be the only likely derivation (see also muir-chlach)
muir-chlach	shingle beach	mura-chlach, bura-chlach (mul-chlach, mol-chlach)	muir "sea" + cloch "stone"	Old Irish	A little conjectural but considering muir-bhuachaille above, muir- seems to be the only likely derivation. The forms mol-chlach/mul-chlach could be either down to a different derivation vial mol shingle or they're also muir- with the mol-/mul- being a form of folk etymology.
naosg	snipe	faosg	naescu "snipe"	Old Irish	*aosg oddly enough does not seem to be attested
neamhanan	tormentil	neamhanan, eamhan, leamhnach	niamnait	Old Irish
neòinean	daisy	nòinean, neòineag, neònan, neònag, eòinean, eòineag, eòintean, feòinean	nóin	Old Irish	Ultimately from Latin nōna (hōra), the ninth hour which coincided with midday and thus came to mean noon. So the derivation in terms of meaning is very similary to that of daisy which comes from Old English dæġes ēage day's eye i.e. the sun.
peucag	peacock	eucag	peacock	Scots/English
pullag	pulley	fullag/fulag and ullag/ulag	pulley	Scots/English	though there's probably two different roots here, one derived via English 'pulley' and the other via the root ullamh 'ready'.
sail-chuach	related to salach, so something like "filthy bowl" originally	dail-chuach, fail-chuach	sal- + cuach "violet"	Old Irish	Presumable we're looking at an t-sail-chuach giving an dail-chuach via eclipsis and via an unattested *tail-chuach » *ail-chuach we get fail-chuach. Ay dios...
seillean	bee	teillean	?		the root of this word is not clear.
triullasg	astray/awry	friollasg/iollasg	?		don't ask me what the root is

When it comes to verbal nouns, rebracketing can lead to some interesting ... detours due to the historic -g in ag/a':

• ràdh/gràdha, selling
• reic/creic, selling
• ulfhart(aich)/gulfhart(aich)/sgulfhartaich, howling

Now in theory it's conceivable that a word like gulfhart could morph into something even weirder such as *dulfhart (via /g/ » /ɣ/ » /d/) which could conceivably result in *tulfhart (via re-analysis of *dulfhart as *an tulfhart) but to date, I haven't spotted one. But I'll keep looking. But sgulfhartaich where there g from the particle ag has picked up a prosthetic s- isn't bad for starters.

So English may excel at punning but there's nothing quite like the interplay between lenition, nasalization and rebracketing in Goidelic languages to delight the phonologist and to drive etymologists to substance abuse...

How about without?

Sure, that works too, depending on the combination of sounds, an offending initial can also just get dropped:

Source	Meaning	Gaelic	Irish	Manx
Latin habitus	dress, attire	eabaid/abaid	aibíd
Latin hecticus	hectic (in the medical sense)	èiteag
Latin Hilarius	(name)	MacEalair
Latin hora	hour	uair	uair	oor
Norse haf	sea, ocean	abh
Norse hattr	hat	ad	háta (prob. via Middle English hat)	edd
Norse hábura	rowlock		adhbhar
Norse hálsa	slacken	allsadh/abhsadh
Norse Hildulfr		Iondolbh
Norse hlunnr	oar-handle	lunn
Norse hǫmull	swingle-tree	amall
Norse hóss	river mouth	ós
Norse hrams-á	ramsons river			Rhumsaa
Norse hrókr	croaker; rook	ròc(as)	rúcach
Norse hrosshvalr	walrus	rosualt/ruashual/rochuaid	rosualt
Norse hrúga	peat-stack	ruc
Norse hrukka	wrinkle	roc, rug	roc, ruc
Norse hrútr	ram	rùt(a)
Norse húfr	hull		ub (prow)
Norse ulfr	wolf	uilbh/ulbh
Norse ulfr	wolf	ulfhart (possibly)
Norse vindáss	windlass	unndais	undás, unlas
English haddock		adag	cadóg/hadóg/feadóg	addag
English handspike		annspach/annspeic
English Harlaw	hard + law	Àrla
English harness		àirneis
English harpy		arpag	airp
English herald(ry)		earraid(eas)
English hood			úda
English Hugo			Úgha
English Judas			údas

Most likely the Gaelic words for St Kilda and St Kildans also belong in this category. There is Hiortach, Iortach and Tiortach but nobody is exactly sure as to which one is the underlying one. Oddly enough, there is no attested form with a C- or Ch- (odd because one of the main theories of the origin of the place names revolves around the Norse word kelda "well, spring" - though there are other, phonetically more convincing ones such as the theory of it being linked to Norse hjǫrtr "hart, stag").

And yes, Welsh is just as much fun

Because this remodelling of sounds is not even exclusively Goidelic. Welsh and the other Brythonic languages do it too, with their own twist (having 3 mutations helps): On occasion, even the permissible fricative f- turns into, well, other stuff, through the same process:

Source	Meaning	Welsh	Cornish	Breton
Latin februārius	February	Chwefror	Hwevrel	C'hwevrer
Latin habēna	rein	afwyn
Latin ianuārius	January		Genver	Genver
Latin mespilum	medlar			mesper, gwesper
Latin vāgina	sheath	gwain	gweyn	gouin
Latin venēnum	poison	gwenwyn		gwenon
Latin Veneris	Friday	Gwener	(dy') Gwener	Gwener
Latin verbum	verb	berf
Latin vestis	vest	gwest
Latin vīcus	village	gwig		gwig
Latin vīnum	wine	gwin	gwin	gwin
Latin vīpera	viper	gwiber		gwiber
Latin viridis	green	gwyrdd	gwer	gwer, gwyrdh
Latin vitrum	glass	gwydr	gweder	gwer
Latin vīverra	ferret	gwiwer		gwiñver
Anglo-Saxon wealhstōd	interpreter	gwalstod
Norman French vilein	peasant	milain/bilain		bilen
English vicar		bicar
English quail		chwail
English ware(s)		gwâr	gawara	gwara
English waste		gwast	gwastya	(v.) gwastañ
English wipe		chweip

And French, Italian, Spanish and so on as a matter of fact

Isn't that just a waffle?

Even outside Celtic this is not unheard of. People just don't think about it.

Many Romance languages borrowed foreign words (especially Frankish words, Frankish being a West-Germanic language which was eventually supplanted by Old Dutch and Old French) beginning with /w/ with a /g/ and /θ/ with a /t/. Basque, due to a lack of f at one point, strengthened loanwords from Romance languages to b or p:

Language	Meaning	Borrowed into	Word borrowed as & meaning
Frankish *þorp	settlement; gathering of people	French	trop (too much)
Frankish *þwahilu	cloth	French	toaille (towel)
Frankish *wad	ford	French	gué (ford)
Frankish *waddi	pledge	French	gage (pledge)
Frankish *waidon	(v.) graze	French	guéder (v. sate)
Frankish *waigaro	a lot	French	guère (hardly)
Frankish *wafel	(n.) waffle	Old French > French	walfre > gaufre
Frankish *wahtōn	(v.) watch (for)	French	guetter, guette (watch for)
Frankish *waidanjan	get food	Italian	guadagnare (earn)
Frankish *walha	non-Germans, foreigners	French	Gaule (Gaul*) and via French also Spanish Gales (Wales)
Frankish *walhlaup	(v. gallop)	French	galoper (v. gallop)
Frankish *want	glove	Catalan	guant (glove)
Frankish *wardon	(v.) guard	Italian	guardare (earn)
Frankish *warjan	protect	French	guérir, guérison (v. cure)
Frankish *warjan	protect	Italian	guarire (get better)
Frankish *warnjan	protect	Italian	guarnire (v. garnish)
Frankish *waskan	(v.) wash	Italian	guazzare (v. wallow)
Frankish *wastil	nourishment	French	gâteau (cake)
Frankish *wenkjan	vacillate	French	guincher (v. boogie)
Frankish *werra	war	French	guerre, guerroyer (war)
Frankish *weron	adorn	French	guirlande (garland)
Frankish *wespa	wasp	French	guêpe (wasp)
Frankish *wid-malve	marshmallow	French	guimauve (marshmallow)
Frankish *wigila	ruse	French	guile (deception)
Frankish *wīhsela	sour cherry	French	guigne (sour cherry)
Frankish *Willahelm	William	French	Guillaume (William)
Frankish *wimpil	headscarf	French	guimpe (wimple, gimp etc)
Frankish *wingjan	wink	French	guigner (v. eye)
Frankish *wīsan	fashion, manner	French	guise (way)
Frankish *wīpan	(v.) wrap	French	guiper, guipure (cover with silk)
Frankish *wītan	know	French	guide, guider (v. guide)
Frankish *withthja (influenced by Latin vitica)	osier	French	guiche (strap)
Frankish *wrakjō	servant, boy	French	garçon (boy)
Frankish *wrist	instep	French	guêtre (gaiter)
French voiture	car	(Northern) Basque	boitura (car)
Gothic waida	woad	French	guède (v. dye)
Gothic wīmon	leap	French	guimbarde (Jew's harp)
Middle High German Welfen	Guelphs	Italian	Guelfi (dynasty)
Middle High German Wibellingen	Ghibelline	Italian	Ghibellini (dynasty)
Latin faba	bean	Basque	baba (bean)
Latin fāgu	beech	Basque	pago/bago (beech)
Latin ficu	fig	Basque	piku (fig)
Latin fīlum	thread	Basque	biru (thread)
Latin fortis	strong	Basque	bortitz (strong)
Latin fricātū	rubbing down	Basque	perekatu (v. caress)
Latin Vascōnēs	Basques	French/Gascon	Gascon, incl. the derived surname Gascoigne
Latin vigilāre	(v.) watch	Basque	begiratu (watch)
Latin vīpera	snake	French	guivre, givre (snake)
Latin viscum (influenced by Frankish *wīhsala)	mistletoe	French	gui (mistletoe)
Latin voluntātem	will	Basque	borondate (will)
Norse vík	bay	French	guichet (gate, wicket etc)
Norse vinda	(v.) wind	French	guinder (v. hoist)
Nahuatl ahuacamolli**	avaocado sauce	Spanish	guacamole
Nahuatl huacalli	calabash tree	Spanish	guacal (calabash tree)
Nahuatl huaxin	white leadtree	Spanish	guaje (white leadtree)
Nahuatl huexolotl	turkey	Spanish	guajolote (turkey)
Nahuatl huīpīlli	woman's blouse	Spanish	güipil/guaypil (traditional Mexican woman's blouse)
Romance verde	green	Basque	berde (green)
Spanish fraile	brother	Basque	praile (friar)
Spanish Francia	France	Basque	Prantzia (France)
Spanish fundamento	foundations	Basque	puldamentu (foundations)

*The similarity to Latin Gallia is a coincidence, these are two different roots.
**Nahuatl hu represents /w/

I don't want to go overboard with examples but just to make the point, you can find this kind of thing all over the planet. Say European loanwords in Asian languages, often related to /f/ or /v/ sounds which often don't exist in the native sound system or are not allowed in certain positions (initially, finally etc):

Source	Meaning	Target language	Term	Meaning
Dutch fabriek	factory	Indonesian	pabrik	factory
Dutch verboden	prohibited	Indonesian	perboden	prohibit
Dutch voorloper	precursor	Indonesian	pelopor	frontrunner
Dutch vrijman	outlaw	Indonesian	preman	gangster
English bus		Thai	บัส bát	bus
English lift		Cantonese	𨋢 līp	lift, elevator
English offside		Cantonese	噏西 āp sāi	offside
English valve		Cantonese	<span style="color: #6600CC;"閥 faht	valve
French vacances	holidays	Japanese	バカンス bakansu	holidays
Portuguese falso	false	Indonesian	palsu	fake
Portuguese festa	festival	Indonesian	pesta	party
Portuguese varanda	veranda	Indonesian	beranda	veranda
Portuguese veludo	velvet	Japanese	ビロード birōdo	velvet
Portuguese vidro	glass	Japanese	ビー玉 bīdama	marbles
Portuguese viola	violin	Indonesian	biola	violin

Or cheat

Returning to Gaelic, cheating works too of course, just as an aside. This happens particularly with loanwords which have an initial h. Initial h does make a regular appearance in Gaelic - but only in certain grammatical contexts, such as the genitive of feminine nouns or the plural if there's a definite article, for example doras na h-eaglaise or na h-ùbhlan. So in some cases, when Gaelic borrowed words which began with h, it slaps an extra article on which wasn't there in the language it borrowed from. For example:

Source	Meaning	Gaelic
Norse hǫfn	harbour	Na Hann
Norse haugr	mound	Na Hogha Bheag
Norse hérað	type of administrative unit	Na Hearadh
Scots Foxhole		A' Bhogsolla

Often, like in Na Hogha Bheag, you're left with a word which obeys the sound rules of Gaelic but has what looks like shifty grammar (you'd expect *Na Hogha Beaga) which is often a telltale sign of a fricative-initial loanword that was squeezed into a corset to make it sound ok.

But why are there so many place-names with initial h?

Good question. I'm making an educated guess here but I think it's because place names, in the past, mostly occurred in the genitive or dative. Today we put up roadsigns and have lists of place names but that's a fairly recent thing. When you're actually talking 'normally', usually place names don't appear in grammatical isolation like that. And in Gaelic, that puts the place name into a leniting environment. Let's use Glaschu as an example. You can be 'in' Glasgow, which is probably the only non-leniting example: ann an Glaschu. But if you're going to Glasgow, you're a' dol a Ghlaschu and the people or shops of Glasgow are muinntir Ghlaschu and bùithtean Ghlaschu.

So ignoring the modern past-time of writing, even a native Gaelic place name like Toirbheartan would very often have a (spoken) initial /h/: a' dol a Thoirbheartan, muinntir Thoirbheartan and so on. Plus most prepositions one is likely to use in front of a place-name (bho, do (a), tro, ro) cause lenition (the main exception being à but which is, incidentally, less common statistically than the others).

So in context, place names like the examples below don't jar as much in the ears of a native as some others might.

Source	Meaning	Gaelic	Anglicised as
Norse hár + klettr	high + cliff	Hàcleit	Habost
Norse heastr + staðr	horse + farm	Heasta (with nearby Allt na Heasta)	Heaste
Norse horna + fjall	horn + mountain	Hòrnabhal	Horneval
Scots haugh + kirk		Hacraig	Halkirk

Simplifly, simplify

No, languages don't always simplify stuff because otherwise we'd all be going round pointing at stuff saying 'uh' and nothing else. But let's leave that topic for another article. Another thing that effectively hides the origins of many Gaelic words is the simplification of certain sounds, either because this change happened throughout the language anyway or because it was necessary to adjust the sounds to fit the language (like the th > t change above).

For example, a language internal process changed many nd/nt sounds to nn and lt/ld to ll. You're probably aware of the coltas/collas issue. That's just an instance of a language internal simplification of lt to ll, nothing to do with English.

So here are some fun examples:

Source	Meaning	Gaelic	Irish	Manx
Latin argentum	silver	airgead "money"	airgead	argid
Latin candela	candel	coinneal	coinneal	cainle
Latin hic pāx datur	peace*	pòg "kiss"	póg	paag
Norse eyland	island	eilean	oileán	ellan
Old Irish cruind	round	cruinn	cruinn	cruinn
Scots/English doctor		dotair	dochtúir

*part of the old Celtic liturgy where peace was given just before Communion, involving a symbolic kiss

Let's talk about your sister

A particularly bizarre example of this is the Gaelic word for sister because it went through this several times:

1. Indo-European used swesōr.
2. Proto-Celtic used swesūr - Proto-Celtic, as in before Brythonic (Welsh 'n all that) and Goidelic (Gaelic 'n all that) split.
3. Old Irish used siur AND fiur.
   OK, this requires some explaining. In the days before Old Irish, the words for my, your and his already caused lenition. So far so good, but this meant that Proto-Celtic swesūr became hwesūr, just like leniting s today. But as the hw never sat well with the language (a bit like thing becoming fing in London) it soon turned into an f, but our, your (pl.), their didn't, and didn't lenite. So, by the time we arrived at Old Irish, unlenited our/your/their sister had become ar siur, far siur, a siur, but lenited my/your/his sister had become mo fiur, do fiur, a fiur. Yes, slightly confusing.
4. To get from Early Irish fiur/siur to the modern forms, we now have to go via the genitive forms for Gaelic (something which did not happen in Irish - don't sigh). So modern Irish just continues with the old roots using deirfiúr "sister" from deirbh fiur "true sister" and fiúr "kinswoman" (because two words are better than one…).
5. However, Scottish Gaelic grabs the old genitives, fethar/sethar, and muscles them into piuthar. The p appears, of course, because f is quite obviously lenited p. Now why didn't you think of that before? 😂

Anyway, to get back to the point, this is the reason why you will hear some Gaelic speakers pronounce certain words with f and some without. It's a process that's still in the middle of happening, so until the language decides on whether it will stick with eagal or adopt feagal, you can take your pick about which one you prefer. It makes no difference, either way, as both are good Gaelic words.