Words for narrow and related things in Celtic languages.

Words marked with a * are reconstructions.

Proto-Celtic	*koilos = thin
Old Irish (Goídelc)	cáel, cóel = thin, slender, narrow, fine, delicate, strait cáelán = intestine
Middle Irish (Gaoidhealg)	cáel, cóel, cóil = thin slender, narrow, fine, nice, delicate, subtle, exact cáelán, caolan = intestine, gut cáelas = narrows, strait
Irish (Gaeilge)	caol [keːl̪ˠ / kiːlˠ] = thin, slender; fine; narrow; shrill; slender; palatalized; weak, dilute; slight; subtle caolach =osiers, twigs, wicker-/wattle-work caoladán = thin person, thing caoladóir = wicker-worker, basker-maker caolaigh = to become thin, narrow, reduce, dilute caolán = creek, small intestine, catgut caolas = strait, narrows, narrow place, bottleneck
Scottish Gaelic (Gàidhlig)	caol [kɯːl̪ˠ] = narrow, slender; slim, thin, lean, skinny; lanky caolan = colon, intestine, entrail(s), gut(s) caolanach = colonic, intestinal caolraid = alley, vennel, wynd caolaich = make slender, slenderise caolachadh = (act of) narrowing, slenderisation caolas = narrow(s), strait caoladair = wickerworker
Manx (Gaelg)	keyl [keːl] = thin, fine, attenuated, lanky, lean, slim, slight, willowy, gracile, tapered, slender, hairline, narrow, spare, small, drawn out, watery (soup), weak (solution) keyllaghey = narrow (down), narrowing keyley = narrowing, slenderize, thin keylagh = to attenuate, attenuation, slenderize, thin, thinning
Proto-Brythonic	*kʉl = narrow
Middle Welsh (Kymraec)	cul, kul = narrow, lean culaidd, culedd = narrowish, restricted culhau, kulhau = to become narrow, shrink
Welsh (Cymraeg)	cul [kɨːl / ˈkiːl] = narrow, lean, illibera,; bigoted, narrow-minded culaidd = narrowish, restricted culder, culdra = narrowness, leanness culdir = isthmus, narrow strip of land culfa = strait, narrow place, narrow passage culhaf, culhau = to become narrow, shrink, lose fat
Middle Cornish (Cernewec)	cul = narrow, strait culurionein, kylyrion = the bowel
Cornish (Kernewek)	kul [ky:l / ki:l] = narrow kuldremenva = corridor
Breton (Brezhoneg)	kul = narrow

Etymology: from Proto-Indo-European *koyHlo- or *skey- (to dissect, split) [source].

The word kyle, which appears in placenames such as Kyle of Lochalsh (Caol Loch Aillse) is an English version of the Scottish Gaelic caol [source].

Another word for narrow in Breton is strizh [ˈstriːs], which comes from the Latin strictus (tightened, compressed). The English words strict and strait come from the same root [source].

Words marked with a * are reconstructions.

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, Gerlyvyr Cernewec, Lexicon Cornu-britannicum: A Dictionary of the Ancient Celtic Language of Cornwall, Dictionaire Favereau, TermOfis, Le dictionnaire diachronique du breton, Geriafurch, English – ProtoCeltic WordList (PDF), Etymological Dictionary Of Proto Celtic