Words for shoulder and related things in Celtic languages.

Words marked with a * are reconstructions.

Old Irish (Goídelc)	gúala = shoulder
Middle Irish	gúala, gualann, gūaluinn = shoulder gúaillech = shouldered, branched, branching gúaillid = companion, comrade gúalaire = load, burden
Irish (Gaeilge)	gualainn [ˈɡuˑəl̪ˠənʲ] = shoulder guailleadóireacht = (act of) shouldering, swaggering guailleáil = to shoulder, to jostle with shoulder, to lurch with shoulders, to saunter, to swagger guailleálaí = shoulderer, lurcher, saunterer, swaggerer guailleán = shoulder-strap, braces guailleog = epaulet(te) guaillí = companion guaillíocht = companionship
Scottish Gaelic (Gàidhlig)	guaille [guəl̪ˠə] = shoulder guailleach [guəl̪ˠəx] = pertaining to shoulders guailleachan / guailleag [guəl̪ʲəxan / guəl̪ʲag] = shoulder-mantle, shawl gualann [guəl̪ˠən̪ˠ] = shoulder, corner of a house, tack (nautical), brow (geography), shoulder (of mutton), shaft (of a cart), curve of a (ship’s) bow
Manx (Gaelg)	geaylin = shoulder, corner (of house) geayliney = shoulder geaylinagh = shouldered geaylaig = epaulette

Etymology: from Proto-Indo-European *gew- (to bend, curve, arch, vault) [source]. Words from the same roots include cuttlefish, gown, kidney, chitterling and gyration in English, Kuhle (hollow, depression) in German, kiel (keel) in Dutch, gonna (skirt) in Italian, and keula (bow, prow) in Finnish [source].

Proto-Celtic	*skēdos = wing, shoulder
Old Irish (Goídelc)	scíath = wing
Middle Irish	scíath = wing scíathán, sgíthéan, sgiathán = wing scíathánach = winged
Irish (Gaeilge)	sciathán [sˠciəˈhɑ̃ːn̪ˠ] = wing, side, extension, part, piece, arm sciathánach = winged, long-armed An tOileán Sciathánach = Isle of Skye
Scottish Gaelic (Gàidhlig)	sgiath [sgʲiə] =wing, fin sgiathan [sgʲiahan] = small wing sgiathach [sgʲiəhəx] = pertaining to shoulders sgiathadaich [sgʲiəhədɪç] = fluttering sgiathadh [sgʲiəhəɣ] = (act of) flying sgiathag [sgʲiəhag] = little wing sgiathaire [sgʲiə.ɪrʲə] = winger (in sports) An t-Eilean Sgitheanach = Isle of Skye
Manx (Gaelg)	skian [skʲiən] = wing, pinion, mudguard, baffle skianagh = winged, finned, pinioned, winged creature Ellan Skianagh = Isle of Skye
Proto-Brythonic	*skuïð = shoulder
Old Welsh (Kembraec)	iscuid = shoulder
Middle Welsh (Kymraec)	ysgwyt, ysgwyd, ysgỽyd = shoulder ysgwydawc, yscwyddoc, ysgwyddoc = shoulder(-blade), (upper part of) arm, shoulder (meat), having shoulders
Welsh (Cymraeg)	ysgwydd [ˈəsɡwɨ̞ð / ˈəsɡʊi̯ð] = shoulder ysgwyddo = to shoulder, push with the shoulder(s), hustle, jostle, support, shrug ysgwyddiad = standing out, jutting out, projection, protusion, cornice ysgwyddog = shoulder(-blade), (upper part of) arm, shoulder (meat), having shoulders ysgwyddol = scapular, like a shoulder in shape
Old Cornish	scuid =shoulder
Middle Cornish (Cernewec / Kernuak)	scuid, scudh, scoudh, scôdh, scûdh = shoulder scuidlien = a hood
Cornish (Kernewek)	skoodh [sko:ð / sku:ð] = shoulder skoodhya = to assist, back, help, second, support skoodhyans = assistance, help, support skoodhyer = assistant, exponent, fan, supporter
Middle Breton (Brezonec)	scoaz = shoulder
Breton (Brezhoneg)	skoaz [ˈskwɑː(s)] = shoulder, help skoazek = (person) with broad shoulders, killer whale skoazell [ˈskwɑː.(z)ɛl] = help, assistance skoazellañ [skwa.ˈ(z)ɛ.lːã] = to help, to rescue, to subsidize skoazeller = assistant, helper, auxiliary skoaziañ [ˈskwa(z)jã] = to support, to help, to assist

Etymology: unkown, possibly from Proto-Indo-European *skeyd- (to split, divide) [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