Words for high and related things in Celtic languages.

Proto-Celtic	*ardwos = high
Gaulish	*arduenna = high
Old Irish (Goídelc)	ard [ar͈d] = high, height
Middle Irish (Gaoidhealg)	ard = high, elevated, lofty, noble, distinguished, great, proud, difficult, laborious, arduous, height ardach = hilly ardae, airde = height, high place, greatness, nobility, loudness ardaid = rises (against), to rebel ardán = pride, arrogance ardrí = high king
Irish (Gaeilge)	ard [ɑːɾˠd̪ˠ / æːɾˠd̪ˠ] = height, hillock, top, high part; high, tall; loud; ambitious; chief, excellent, noble, advanced ardaigh = to raise, elevate, ascend, carry ardán = small height, platform, stage, stand, terrace, uplift ardrí = high king ardscoil = high school ardú = elevation, increase, exaltation, excitement
Scottish Gaelic (Gàidhlig)	àrd [aːr̪ˠd] = high, lofty, tall; great; loud; chief, eminent, superior, supreme àrd-cheann = superior, chief àrd-chùirt = high court àrd-chiallach = intellectual àrd-ollamh = professor àrd-sgoil = secondary school, high school àrdchadh = raising, elevating, promoting, advancing, raise, elevation, promotion, advancement àrdaichte = raised, upgraded àrdan = arrogance, haughtiness, pride, elevation, eminence, height
Manx (Gaelg)	ard = high, towering, tall, big, loud, height, high place, fell, incline, district, region, direction, compass point, pole ardan = platform, stage, stand ard-chione = head, principal, superior, supremo ardys = exaltation, greatness, height
Proto-Brythonic	*arð = high
Middle Welsh (Kymraec)	ard, art = hill, highland, top; high, upland
Welsh (Cymraeg)	ardd [arð] = hill, highland, top; high, upland ardd-tir = high ground
Middle Cornish (Cernewec)	ard, arth = height, high place arthelath = lordship
Cornish (Kernewek)	ardh = height, high place
Breton (Brezhoneg)	arz = high, elevated

Etymology: from Proto-Indo-European *h₃r̥dʰwós, from *h₃erdʰ- (to increase, grow; upright, high) and *-wós (creates adjectives from verbs) [source].

The Ardennes, a region of forests and hills in mainly in Belgium, Luxembourg, and also in France and Germany, was known as Arduenna Silva in Latin. The first part of the Latin name probably comes from the Gaulish *arduenna, or from the Latin arduus (lofty, high, steep, tall), which comes from the same PIE root [source].

Other words from the same PIE roots include arbor, arduous, orthodox and orthography in English, arbre (tree) in French, árbol (tree, mast) in Spanish, and рост [rost] (growth, increase, rise, height, stature) in Russian [source].

Proto-Celtic	*ouxselos = high, elevated
Gaulish	*uxelos = high
Old Irish (Goídelc)	úasal = high
Middle Irish (Gaoidhealg)	úasal = high, lofty, noble, honourable, noble
Irish (Gaeilge)	uasal [ˈuəsˠəlˠ] = noble, high-born; gentle, gentlemanly; precious, fine; (of place) sacred to the dead; hallowed; enchanted, inhabited by fairies
Scottish Gaelic (Gàidhlig)	uasal [uəsəl̻ˠ] = noble, highminded, genteel
Manx (Gaelg)	ooasle = aristocratic, lofty, illustrious, esteemed, gentlemanly, patrician, honourable, dignified, lordly, magnificent, classy, respected
Proto-Brythonic	*ʉxel [ʉˈxɛːl] = high, elevated
Middle Welsh (Kymraec)	vchel, uchel = high, tall
Welsh (Cymraeg)	uchel [ˈɨ̞χɛl / ˈiːχɛl /ˈɪχɛl] = hill, tall; high(-ranking), exalted, important, solemn, sublime, splendid, excellent, noble, stately, respectable, commendable; proud, haughty, arrogant, presumptuous, snobbish uchelaf, uchelu = to raise, heighten, exalt, increase uchelaidd = exalted ucheldeb = highness uchelder = high place, height, tallness, altitude, climax ucheldir = highland, upland, high place, the Highlands ucheldra = highness, height, glory, majesty, authority ucheledig = elevated, sublime, noble
Middle Cornish (Cernewec)	uchel = high, lofty, towering uchelder = height, highness, loftiness uchelle = to make high, exalt, hallow, sanctify
Cornish (Kernewek)	ughel [‘ʏhɛl / ‘ɪʍɐl] = high, grand, loud, tall ugheldas = patriarch ughelder = height, loudness, tallness, volume ughella = higher, superlative ughelor = noble ughelvam = matriarch
Old Breton	uchel, uhel = high
Middle Breton	vhel, uhel, uc’hel = high uhelaat = to raise, elevate uheladur = raising, heightening uhelañ = highest, summit uheldad = patriarch uhelded = height, greatness, loftiness uhelegezh = ambition, pride
Breton (Brezhoneg)	uhel = high, upstream, uphill uhelaat = raising uhelder = altitude uhelded = height, greatness uheldiad = dignitary uhelvrud = prestige

Etymology: from Proto-Indo-European *h₃ewps- (height) [source].

Words from the same PIE root include subtle and supine (lying on its back) in English, sotto (down, underneath, below) in Italian, arduous, orthodox and orthography in English, arbre (tree) in French, árbol in Spanish, and рост [rost] (growth, increase, rise, height, stature) [source].

Words marked with a * are reconstructions.

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, Gerlyvyr Cernewec, Dictionaire Favereau, TermOfis, Le dictionnaire diachronique du breton, Geriafurch, English – ProtoCeltic WordList (PDF), Etymological Dictionary Of Proto Celtic