Words for back in Celtic languages.

Old Irish (Goídelc)	druimm [drumʲ] = back, ridge
Irish (Gaeilge)	droim [d̪ˠɾˠiːmʲ / d̪ˠɾˠɪmʲ] = back, ridge, carapace, wave
Scottish Gaelic (Gàidhlig)	druim [drɯim] = back, ridge, keel, backline, camber
Manx (Gaelg)	dreeym = back, ridge, hillside, down, terrace, shed, camber, saddle, fret, arch, edge

Sources: Wiktionary, Am Faclair Beag, Online Manx Dictionary, Teanglann.ie

Proto-Celtic	*kebno- = back
Gaulish	Cebenna [keˈben.na] = ridge, height, and name of the mountins now known as the Cévennes
Middle Welsh (Kymraec)	keuen [ˈke.ven] = back
Welsh (Cymraeg)	cefn [kɛvn / ˈkeːvɛn] = back, support, ridge, middle, center
Cornish (Kernewek)	keyn [kɛɪn / kəɪn] = back, keel, ridge
Breton (Brezhoneg)	kein [ˈkɛjn] = back, keel, binding, convexity

Words marked with a * are reconstructions.

Sources: Wiktionary, Geiriadur Prifysgol Cymru, Gerlyver Kernewek, Dictionnaire Favereau