Here are some words for knee, and related words, in Celtic languages.

Proto-Celtic	*glūnos = knee
Old Irish (Goídelc)	glún [ɡluːn] = knee
Irish (Gaeilge)	glúin [ɡl̪ˠuːnʲ / ɡlˠuːnʲ] = knee, generation, step (in a series), node caipín glúine, gealacán glúine = kneecap, patella bean ghlúine = midwife
Scottish Gaelic (Gàidhlig)	glùin [ɡl̪ˠuːnʲ] = knee, generation, lap, joint (of boat) copan na glùine / cnap-glùine / falman = kneecap, patella lùbadh-glùin = curtsey bean-ghlùine = midwife
Manx (Gaelg)	glioon [ɡlʲuːnʲ] = joint, crooked timber, knee rollian ny gliooney = kneecap, patella moddey glioon = lap dog, toy dog glioon-lhoobey = genuflect, genuflection
Proto-Brythonic	*glin = knee
Welsh (Cymraeg)	glin [ɡliːn], pen (g)lin [pɛnˈ(ɡ)liːn] = knee clwyd y glin = kneecap, patella llinach = lineage, genealogy, pedigree, ancestry, stock, kindred, offspring, succession, generation camu glin = to curtsey
Cornish (Kernewek)	glin [ˈɡliːn] = knee padell bedn glin / penn glin = kneecap, patella
Breton (Brezhoneg)	glin [ˈɡliːn] = knee koubloù = kneecap, patella

Etymology: from the Proto-Indo-European *ǵónu (knee).

From the Proto-Celtic word *glūnos we also get the Proto-Celtic word *genwā ((river) bend), which is the root of the names Geneva, Genoa.

The word knee comes from the same PIE root, via the Middle English kne, from the Old English cnēow, from the Proto-West Germanic *kneu, from the Proto-Germanic *knewą.

The word genuflect (to bend the knee) also comes from the same PIE root, via the Latin genū (knee) and flectō (to bend) [source].

Sources: Wiktionary, Am Faclair Beag, MacBain’s Dictionary, In Dúil Bélrai English – Old-Irish Glossary, teanglann.ie, On-Line Manx Dictionary, Geiriadur Prifysgol Cymru, Gerlyver Kernewek, Dictionnaire Favereau, TermOfis