Words for strength and related things in Celtic languages.

Words marked with a * are reconstructions.

Proto-Celtic	*brīgos = strength *brīga = power, worth
Gaulish	*brīgos = strength
Old Irish (Goídelc)	bríg [bʲrʲiːɣ / bʲrʲiːɣʲ] = force, power, value, virtue, strength, vigour, vitality brígach = powerful, mighty, strong brígmar = powerful
Middle Irish (Gaoidhealg)	bríg, brigh = power, strength, force, authority, vigour, virtue, might, value, worth, advantage, meaning brígach = powerful, mighty, forceful brígda = strong, vigorous brígmar = powerful, strong, vigorous, lively, efficacious brígrad = power, force
Irish (Gaeilge)	brí [brʲiː] = strength, vigour; force, significance; influence, merit bríoch = strong, vigorous, efficacious bríochmar = strong, viorous bríochtach = strong, vigorous person
Scottish Gaelic (Gàidhlig)	brìgh [brʲiː] = essence, gist, matter, pith, purport, substance; meaning, sense, significance; point (of an argument); energy, force brìghealachd [brʲiː.əLəxg] = substance, juiciness, pithiness, significance (in mathematics) brìgheil [brʲiː.al] = meaningful, significant brìoghmhor [brʲiː(v)ər] = meaningful, energetic, substantial, pithy
Manx (Gaelg)	bree = power, energy, stamina, vigour, virtue, initiative, validity, animation, inwardness, glow, exhalation, drift, essence, gist, effect breeagh = inspiring, vigorous breeoil = dynamic, energetic, essential, impulsive, valid, vibrant, vigorous, active, powerful
Proto-Brythonic	*briɣ = honour, dignity (?)
Middle Welsh (Kymraec)	bri = honour, dignity, reputation, fame
Welsh (Cymraeg)	bri [briː] = honour, dignity, reputation, fame, prestige, esteem, power, authority, importance, value, popularity briaeth = honour, dignity briol = honourable, reverent, dignified
Middle Cornish (Cernewec)	bry = account, value, worth, price
Cornish (Kernewek)	bri = distinction, esteem, importance, relevance, reputation, value, prominence fowt bri = insignificance heb bri = irelevant
Old Breton (Brethonoc)	bri = importance, weight, authority
Middle Breton (Brezonec)	bry = respect, consideration
Breton (Brezhoneg)	bri [briː] = dignity, honour, respect, consideration brient = privilege, prerogative brientek [bri.ˈɛn.tek] = privileged brientin = = privileged, aristocrat

Etymology: possibly the Proto-Indo-European *bʰrḗǵʰ-o-s, from *bʰerǵʰ- (to rise, ascend, to be elevated, up high). Words from the same roots, via Gaulish, include brio (vigour, vivacity) in English, briu (energy, push, courage) in Catalan, brio (vivacity, liveliness) in Italian, brío (vigour, mettle, zeal) in Spanish, and brio (brilliance, panache) in French [Source].

Words from the same PIE roots include barrow, burrow, bury, effort, force and fort in English, and brenin (king), bwrw (to hit, strike, cast) in Welsh [Source].

Proto-Celtic	*nertom = strength, power
Celtiberian	Nerto- = strength (?)
Gaulish	Nerto- = strength (?)
Old Irish (Goídelc)	nert [n͈ʲer͈t] = power, strength nertaid [ˈn͈ʲer͈tɨðʲ] = to strengthen
Middle Irish (Gaoidhealg)	nert, nnert, nirt = strength, might, power, ability, significance nertaid = to strengthen, confirm, exhort, urge nertaigid = to strengthen, confirm, encourage, grow strong, side with support
Irish (Gaeilge)	neart [nʲaɾˠt̪ˠ / n̠ʲæɾˠt̪ˠ] = strength, force, power, ability, plenty neartaigh = to strengthen neartaitheach = strengthening, reinforcing neartaitheoir = strengthener, abettor neartmhaire = vigorousness, strength neartmhar = strong, vigorous, powerful neartú = strengthening, reinforcement, support
Scottish Gaelic (Gàidhlig)	neart [̪nʲɛRʃd] = might, strength, force, energy, power, (alcoholic) proof, majority, most neartachadh [n̪ʲɛRʃdəxəɣ] = strengthening, building up, making strong neartaich strengthen, build up, make strong neartaiche = strengthener, emphasiser neartail = mighty, powerful, emphatic, forcible neartmhor = mighty, powerful, emphatic, forcible neartmhorachd = robustness, vigour
Manx (Gaelg)	niart = strength, power, force niartaght = strength niartal = cogent, forcible, mighty, potent, powerful, strong niarteyder = strengthener
Proto-Brythonic	*nerθ [nɛrθ] = strenght, force, power
Old Welsh (Kembraec)	nerthi = to strengthen, reinforce, fortify
Middle Welsh (Kymraec)	nerth, nyrth = force, strength, power, might nerthawc, nerthawg, nerthog = strong, mighty, powerful nerthawl, nerthol = strong, mighty, powerful nerthu = to strengthen, reinforce, fortify nerthyd, nerthwr = fortifier, supporter, helper
Welsh (Cymraeg)	nerth [nɛrθ] = force, strength, power, might, energy, vigour, hardness nerthedig = strong, strengthened nerthog = strong, mighty, powerful, potent nerthol = strong, mighty, powerful, potent nerthu = to strengthen, reinforce, fortify, aid, help, exhort, urge nerthwr, nerthydd = fortifier, supporter, helper
Middle Cornish (Cernewec)	nerth = might, power, strength, force
Cornish (Kernewek)	nerth [nɛrθ] = energy, force, might, power, strength nertha = to strengthen nerthek = energetic, powerful, robust
Old Breton (Brethonoc)	nerth = force, energy
Middle Breton (Brezonec)	nerz, nerh = force, energy nerzus, nerhus = vigorous, robust, energetic
Breton (Brezhoneg)	nerzh [nɛrs] = force, energy nerzhan nerzhañ [ˈnɛrzan / ˈnɛrzã] = to strengthen, reinforce nerzhder = vigueur nerzhek = drastic nerzhekaat = to give energy nerzhelour = dynamic nerzhus = vigorous nerzhusaat = to strengthen oneself

Etymology: possibly the Proto-Indo-European *h₂ner-to- (virile, strong), from *h₂nḗr- (man, vital, energy). Words from the same roots include njer (man, human, person) in Albanian, άντρας [ˈandras] (man, husband) in Greek, noras (wish, desire, will, intention) in Lithuanian, nêr (lord, chief) in Welsh, and the name Nero [Source].

Old Irish (Goídelc)	láitir = strong, powerful
Middle Irish (Gaoidhealg)	láitir, laitir [ˈl͈aːdʲərʲ] = strong, powerful
Irish (Gaeilge)	láidir [ˈl̪ˠɑːdʲəɾʲ / ˈl̪ˠaːdʲəɾʲ] = strong, powerful, durable, tough, solid, forcible, loud láidreacht = strength láidrigh = to strengthen
Scottish Gaelic (Gàidhlig)	làidir [l̪aːdʲɪrʲ] = strong, potent, emphatic, robust, substantial làidireachd = strength làidireach = strong, potent, emphatic làidrich = make strong, strengthen
Manx (Gaelg)	lajer = strong, potent, vigorous, hard, heavy, powerful, stark lajeragh = to strengthen, strengthening lajerid = potency, powerfulness, strength, vigour lajerys = cogency, force, might, prowess, strength

Etymology: unknown [Source].

Irish (Gaeilge)	cadrán = hardness, stubborness, obstinacy cadránta = hard, unfeeling, stubborn, obstinate cadrántacht = hardness, stubborness, obstinacy
Proto-Brythonic	*kadarn = strong, powerful, mighty (?)
Middle Welsh (Kymraec)	cadarn, kadarn, katarnn = strong, powerful, mighty cadarnhau, kadarnaha = to strengthen, secure, fortify, safeguard kedernyt, kedernit, cedernyt = strength, power, potency, might
Welsh (Cymraeg)	cadarn [ˈka(ː)darn] = strong, powerful, mighty, firm, fast cadarnhad = confirmation cadarnhau = to strengthen, secure, fortify, safeguard cadernid = strength, power, potency, might
Middle Cornish (Cernewec)	cadarn = strong, stout, valiant

Etymology: from Proto-Celtic *katus (battle), from Proto-Indo-European *kéh₃tus (battle), from *keh₃- (to fight). Words from the same roots include Hader (dispute, quarrel) and hadern (to quarrel, bicker, struggle) in German, and words for battle in Celtic languages [Source].

Proto-Celtic	*kriɸmos = strong (?)
Proto-Brythonic	*krɨβ̃ = strong
Middle Welsh (Kymraec)	cryf, kryf = strong, powerful, vigorous cryfder = strength, power, might
Welsh (Cymraeg)	cryf [krɨːv / kriːv] = strong, powerful, vigorous, intoxicating, tough, rich, fruitful, intense cryfder = strength, power, might cryfhau = to strengthen
Old Cornish	crif = strong, mighty, vigorous, hardy
Middle Cornish (Cernewec)	cref, crŷf, crif, crev = strong, mighty, vigorous, hardy
Cornish (Kernewek)	krev = forceful, powerful, robust, strong, substantial krevder = intensity, strength
Middle Breton (Brezonec)	cref, creff = strong, powerful
Breton (Brezhoneg)	kreñv [ˈkrẽ(w)] = strong, solid kreñvaat [krẽˈfɑːt] = to strengthen, reinforce kreñvder [ˈkrẽ(w)dɛr] = strength, power, might kreñvlec’h = stronghold, fortress

Etymology: from Proto-Celtic *kriɸ- (body) from Proto-Indo-European *krep- (body). Words for body in Celtic and other languages come from the same roots [Source].

Proto-Celtic	*trexsnos = strong *trexsos = stronger
Gaulish	Trexius, Trexa, Trenus = personal names
Primitive Irish	ᚈᚈᚏᚓᚅᚐᚂᚒᚌᚑᚄ (ttrenalugos), ᚈᚏᚓᚅᚐᚌᚒᚄᚒ (trenagusu) = (?)
Old Irish (Goídelc)	trén [tʲrʲeːn] = strong treise = power, strength
Middle Irish (Gaoidhealg)	trén = strong, powerful, strong man treise = strength, vigour, power
Irish (Gaeilge)	tréan = strong man, warrior, champion, strength, power, intensity, plenty, abundance tréaniarracht = strong, forcible, attempt treise = strength, power, dominance, force, emphasis treisigh = to strengthen, reinforce, fortify treisiúil = strong, forceful, vigorous
Scottish Gaelic (Gàidhlig)	trèine [treːnə] = might, power treise [treʃə] = strength, vigour treun [treːn] = champion, hero, brave, strong, sturdy treunar [treːnər] = strong man, champion, hero; very brave, heroic treunas = might, power treuntachd = boldness, courage treuntas = strength, power, magnanimity
Manx (Gaelg)	trean = brave, firm, heroic, intensive, mighty, stout, valiant treanid = braveness, exploit, feat heroics, might, strength, valour
Proto-Brythonic	*trex = stronger
Middle Welsh (Kymraec)	trech = stronger, mightier, greater, superior, better
Welsh (Cymraeg)	trech [treːχ] = stronger, mightier, greater, superior, better trechedd = supremacy, dominance trechu = to defeat, overcome, overpower
Cornish (Kernewek)	trygh = conquest, victory, superior, triumphant, victorious trygher = victor tryghi = to conquer, vanquish
Middle Breton (Brezonec)	trech = victorious, victory trechy, trechiff = to conquer, prevail, overcome
Breton (Brezhoneg)	trec’h [briː] = superior, victorious, victory trec’hadeg = triumph trec’hadenn = success trec’hedigezh = defeat trec’her = winner trec’hin, trec’hiñ = to conquer, prevail, overcome trec’hus = victorious

Etymology: the Proto-Indo-European *(s)treg- (to be stiff, rigid, strong) or *treg- (strength). Words from the same roots include þróttur (strength, vigour, energy) in Icelandic, and idrott (sport, physical education) in Swedish [Source].

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, Dictionaire Favereau, TermOfis, English – ProtoCeltic WordList (PDF), Etymological Dictionary Of Proto Celtic