Words for sack, bag, stomach in Celtic languages.

Proto-Celtic	*bolgos = sack, bag, stomach
Gaulish	*bolgā = sack, bag, stomach
Old Irish (Goídelc)	bolc = belly, stomach; bag, satchel; bellows
Irish (Gaeilge)	bolg [ˈbˠɔlˠəɡ] = belly, stomach, abdomen; bag; bulge, broad part, middle; bellows bolgach = big-bellied, bulging bolgadán = corpulent person or animal bolgán = bubble, bulb, air-bladder (of fish), puff-ball, windbag (of person) bolgchaint = ventriloquism
Scottish Gaelic (Gàidhlig)	bolg [bɔl̪ˠɔg] = blister, bulge, (light) bulb bolgach = abounding in bags/blisters, bellied, bulging, jutting, knobby bolgadh = billowing, puffing up/out, ballooning, swelling bolgan = (plant) bulb, lightbulb
Manx (Gaelg)	bolg [bolɡ] = stomach, abdomen, belly, tummy, corporation, bilge, bowl (of lamp) bolgagh = abdominal, bellied, billowy, blisterm bulging, puffy bolgan = bladder, blister, sac, vesicle, bubble
Proto-Brythonic	*bolɣ = sack, bag, stomach
Old Welsh	bolg = belly, stomach
Middle Welsh (Kymraec)	bol, boly = belly, stomach
Welsh (Cymraeg)	bol [bɔl] = belly, paunch, abdomen, stomach, bowels; tripe; appetite, desire, gluttony, liking; womb; swelling, bulge, surface, side bolio = to gorge, belly, swell, bulge, swagger boliog = (big-)bellied, corpulent, swollen, bulging, pregnant
Cornish (Kernewek)	bolgh [bɔlx] = breach, gap, opening bolghen = boll, capsule
Middle Breton	bolc’h = flax pod
Breton (Brezhoneg)	bolc’h = flax pod

Etymology
From Proto-Indo-European *bʰólǵʰ-os (skin bag, bolster), from *bʰelǵʰ- (to swell) [source].

English words from the same PIE root include bellows, belly, and bolster, via Old English and Proto-Germanic, billow via Old Norse and Proto-Germanic, foolish and folly via Old French and Latin [source], and bulge, budge and budget via Old French, Latin and Gaulish [source].

Words marked with a * are reconstructions.

Sources: Wiktionary, Am Faclair Beag, Online Manx Dictionary, Teanglann.ie, Geiriadur Prifysgol Cymru, Gerlyver Kernewek, Dictionnaire Favereau