Words for bitter, sour and related things in Celtic languages.

Words marked with a * are reconstructions.

Proto-Celtic	*swerwos = bitter
Old Irish (Goídelc)	serb [sʲerv] = bitter, bitterness serbae = bitterness
Middle Irish (Gaoidhealg)	serb = bitter, hurtful, grievous, disagreeable, harsh, discordant serbae, seirbe = bitterness, asperity
Irish (Gaeilge)	searbh [ˈʃaɾˠəvˠ / ˈʃarˠu(ː)] = bitter, sour, acid searbhaigh = to sour, embitter, become bitter searbhán = bitter person, bitter herb, bitters searbhánta = bitter, acrid searbhas = bitterness, sourness, acidity searbhasach = bitter, acrimonious searbhóg = bitter person, bitter woman, bitter drink searbhú = embitterment
Scottish Gaelic (Gàidhlig)	searbh [ʃɛrɛv] = bitter, sour, tart, disagreeable, acidic searbh-chainnt = sarcasm searbh-ghlòr = cacophony searbh-nhilis = bitter-sweet duine searbh = disagreeable person fion searbh geal = dry white wine leann searbh = bitter (ale) ’s searbh an fhirinn = the truth hurts
Manx (Gaelg)	sharroo = acid, acrid, acrimonious, bitter, cutting, embittered, sardonic, sour, tart, unpalatable, vitriolic sharrooaghey = to embitter sharrooane = bitters sharrooid = bitterness lhune sharroo = bitter (ale/beer)
Proto-Brythonic	*hwerw = bitter
Middle Welsh (Kymraec)	chuerv, chuerw, chwerw, chuero, chỽerw = bitter, acrid, painful, harsh chwerwy, cwherwa = to become bitter, be displeased chwerwder, chweruder = bitterness, sourness, acerbity chwerwed, chỽerỽed = bitterness, sourness, acerbity, sharpness, tartness
Welsh (Cymraeg)	chwerw [ˈχwɛru/ˈχweːru] = bitter, acrid, painful, harsh, rough, severe, sharp, surly, hurtful, angry, irate, spiteful, cross, cruel, sorrowful chwerwaf, chwerwi = to become bitter, be displeased chwerwaidd = bitter, sharp chwerwder = bitterness, sourness, acerbity chwerwedd = bitterness, sourness, acerbity, sharpness, tartness
Middle Cornish (Cernewec)	chuero, wherow = bitter, cruel, hardhearted
Cornish (Kernewek)	hwerow = bitter, harsh, sharp hwerowder = acrimony
Old Breton (Brethonoc)	hueru = bitter
Middle Breton (Brezonec)	hueru, fero, huerhue = bitter hueruentez = bitterness
Breton (Brezhoneg)	c’hwerv [χwɛʁw] = bitter c’hwervaat = to make or become bitter c’hwervded, c’hwervder, c’hwerventez, c’hwervoni = bitterness

Etymology from Proto-Indo-European *swer- (to ache, to fester, wound, injury). Words from the same root include sword in English, chwarren (gland, knot in wood, boil, ulcer) in Welsh, zweren (to swear, pledge, declare under oath) in Dutch, and schwären (to fester, hurt, suppurate) in German [source].

Proto-Celtic	*gʷereti, gʷorti- = bitter
Old Irish (Goídelc)	goirt [ɡor͈ʲtʲ] = bitter, salty
Middle Irish (Gaoidhealg)	goirt = hungry, starved, bitter, sour, salt, sharp, keen
Irish (Gaeilge)	goirt [ɡʌɾˠtʲ / ɡɔɾˠtʲ] = salt, saline, salted, bitter goirte = saltiness, salinity, brackishness, bitterness goirteamas = saltiness, bitterness, salt food goirtigh = to salt, pickle
Scottish Gaelic (Gàidhlig)	goirt [gɔr̪ˠʃdʲ] = sore, painful, sour, salted goirteas [gɔr̪ˠʃdʲəs] = pain, ache, painfulness goirtear [gɔr̪ˠʃdʲər] = miser, mean/stingy person goirte [gɔr̪ˠʃdʲə] = soreness, painfulness, sourness, acerbity, saltiness goirteachadh [gɔr̪ˠʃdʲəxəɣ] = hurting, afflicting, acidifying, making sour, leavening
Manx (Gaelg)	gort = acid, bitter, brackish, rank, vinegarish, vinegary, sour, hurt, poignant, acrid, acrimony gortaghey = hurt, hurting, maim, pain gortagh = beggarly, frugal, grudging, hurt, meagre, miser

Etymology from Proto-Indo-European *gʷorti-, from *gʷʰer- (warm, hot). Words from the same root include barmy, furnace, gore, thermal and warm in English, and garstig (rude, nasty, beastly, foul) in German [source].

Sources: Wiktionary, Am Faclair Beag, Online Manx Dictionary, Teanglann.ie, eDIL – Electronic Dictionary of the Irish Language, In Dúil Bélrai English – Old Irish glossary, Geiriadur Prifysgol Cymru, Gerlyver Kernewek, Dictionaire Favereau, TermOfis, English – ProtoCeltic WordList (PDF), Etymological Dictionary Of Proto Celtic