Words for back, ridge and related things in Celtic languages.

Words marked with a * are reconstructions.

Proto-Celtic	*drosman =
Old Irish (Goídelc)	druimm [ˈd̪ɾumʲ] = back, ridge druimm tar ais = backwards, upside down druimm tar druimm = topsy-turvy, upside down
Middle Irish (Gaoidhealg)	druim(m) = back, surface, ridge, hill druimne = back, elevation, ridge, hill drumman = humped, back, hillock
Irish (Gaeilge)	droim [d̪ˠɾˠiːmʲ / d̪ˠɾˠɪmʲ] = back, ridge, carapace, wave droimeann = white-backed, white-backed cow droimfhada = long-backed, long-ridged droimín = low ridge, slight elevation, saddle, arch (in back) droimíneach = tiny humpbacked person droiminne = white colour, white streak, on back droimne = ridge droimeach = black-backed gull, ridged, undulating, arched, convex
Scottish Gaelic (Gàidhlig)	druim [drɯim] = back, ridge, keel, backline, camber druimeag [drɯmag] = small ridge, small load, lump, hump druimean [drɯman] = (mountain) saddle druimneach [drɯimnəx] = dorsal, pertaining to backs, ridged
Manx (Gaelg)	dreeym = back, ridge, hillside, down, terrace, shed, camber, saddle, fret, arch, edge dreeymagh = ridged drommey = dorsal, spinal

Etymology: possibly from Proto-Indo-European (end, boundry), which is also the root of the English word tram, or related to Latin dorsum (back, ridge) [source].

Proto-Celtic	*kebno- = back
Gaulish	Cebenna [keˈben.na] = ridge, height, and name of the mountins now known as the Cévennes
Old Welsh (Kembraec)	ceng, cemn, kewin = back, ridge
Middle Welsh (Kymraec)	keuen, kefneu, kefyn, cefn, cefen [ˈke.ven] = back, ridge kein, cain, ceing = back, ridge cefnai, kefnu, kefnv, cevnu = to turn one’s back upon
Welsh (Cymraeg)	cefn [kɛvn / ˈkeːvɛn] = back, support, ridge, middle, center cefnaf, cefnu = to turn one’s back upon, to forsake, to withdraw, to give way, to finish, to accomplish, to settle, to overcome, to back, to support cefnog = valiant, stout, brave, strong, wealthy, backed, ridged cefnogaeth = support, encouragement, backing cefnogi = to encourage, support, back, strengthen
Middle Cornish (Cernewec / Kernuak)	cein, chein, keyn = back, ridge
Cornish (Kernewek)	keyn [kɛɪn / kəɪn] = back, keel, ridge keynek = convex keyner = full back
Middle Breton (Brezonec)	queyn, quein = back
Breton (Brezhoneg)	kein [ˈkɛjn] = back, keel, binding, convexity keinañ = to lean back, to support, to connect, to put a keel on keineg = having a good back, important, mackerel keinek = having a back keinel = dorsal keinet = backed

Etymology: possibly related to Proto-Celtic *kambos (crooked, bent) [source].

Old Irish (Goídelc)	escir = ridge, elevation
Middle Irish (Gaoidhealg)	e(i)scir = ridge, elevation
Irish (Gaeilge)	eiscir [ˈeʃcəɾʲ] = glacial ridge, esker (a long, narrow, sinuous ridge created by deposits from a stream running beneath a glacier)
Scottish Gaelic (Gàidhlig)	aisgeir = rocky mountain, ridge of high mountains
Manx (Gaelg)	sker = ridge, crag, rockface, skerry, sea rock
Old Welsh (Kembraec)	escier = limb
Middle Welsh (Kymraec)	eskeir, esgeir, ysgeir, esgair = leg, shank, ridge, mountain spur
Welsh (Cymraeg)	esgair, ysgair = leg, shank, limb, ridge, mountain spur
Old Cornish	(el)escher = the shank, shin bone
Middle Cornish (Cernewec / Kernuak)	elesc(h)er = the shank, shin bone
Cornish (Kernewek)	esker = esker (long hill of outwash formed at glacier snout) el esker = shin
Breton (Brezhoneg)	esker = (carpenter’s) leg, knee

Etymology: unknown [source].

Sources: Wiktionary, Etymological Dictionary Of Proto Celtic, In Dúil Bélrai English – Old Irish glossary, eDIL – Electronic Dictionary of the Irish Language, Teanglann.ie, Am Faclair Beag, An etymological dictionary of the Gaelic language, Fockleyreen: Manx – English Dictionary, Online Manx Dictionary, Gaelg Corpus, Geiriadur Prifysgol Cymru, Lexicon cornu-britannicum : a dictionary of the ancient Celtic language of Cornwall, Gerlyver Kernewek, Devri : Le dictionaire diachronique du breton, Geriafurch, TermOfis