Words for to burn and related things in Celtic languages.

Words marked with a * are reconstructions.

Proto-Celtic	*lasketi = to burn *laxsaros = shining, burning *losk-os = ?
Old Irish (Goídelc)	loscid = to burn loiscnech = burning loscud = burning, fire fo·loisci = to scorch
Middle Irish (Gaoidhealg)	loscid, laiscedh, losced = to burn, consume by fire, lay waste by fire, inflame, afflict, revile
Irish (Gaeilge)	loisc = to burn, fire, scorch, sear, sting – (verb noun = loscadh) loiscadh = burning, searing, scorching, stinging loisceanta = flaming, fiery loisceantacht = fieriness loisceoir = incinerator loiscneach = firing, firewood, caustic, burning, stinging, pain, scorched, arid, fiery, fierce loiscní = fiery intensity, fieriness, aridity, burned, parched, condition
Scottish Gaelic (Gàidhlig)	loisg [l̪ˠɔʃɡ̊ʲ] = to burn, inflame, consume, parch, singe; scorch, scald; fire (a gun) – (verb noun = losagadh) loisgeach [l̪ˠɔʃgʲəx] = burning, fiery, flaming, incendiary, inflammatory, igneous, caustic, corroding loisgeadair [l̪ˠɔʃgʲədɪrʲ] = burner, incinerator loisgte [l̪ˠɔʃdʲə] = burnt, scorched, scalded, drunk
Manx (Gaelg)	losht = to burn, burn up, burn away, incinerate, cremate, fire, alight – (verb noun = lostey) loshtee, loshtit = igneous, burnt loshteyr = arsonist, firebrand, incendiarist loshteyder = arsonist, burner, incinerator yn-loshtey = combustible
Middle Welsh (Kymraec)	llosc = burning, scalding, burn lloscadwy = combustible, burning, fiery, scorching lloscy, llosci, llosgi = to be on fire, blaze, be alight lloscetic, lloskedic, llosgedic, lloscedic = burnt, charred, burning, fiery
Welsh (Cymraeg)	llosg = burning, scalding, burn, scald, fire, conflagration, blaze, arson, singeing, inflammation llosgadwy = combustible, burning, fiery, scorching llosgi [ˈɬɔskɪ / ˈɬɔski] = to be on fire, blaze, be alight, be burnt or be fired, burn up, burn down, burn away; be inflamed, smart, sting, be sunburnt; be inflamed with anger, passion, etc llosg(i)edig = burnt, charred, burning, fiery
Middle Cornish (Cernewec)	losc = a burning, inflammation, searing lescy, loscy = to burn, to be burning losow, lusow = ashes
Cornish (Kernewek)	leski [lɛski] = to burn losk = burning, combustion loskadow = flammable loskrias, loskriades = arsonist loskrians = arson loskven = sulphur loskvenydh, loskvena = volcano
Old Breton	lescsit = to burn
Middle Breton (Brezonec)	lisquiff, lesquiff, losquan = to burn losquadur [los.ˈkɑː.dyr] = burning
Breton (Brezhoneg)	leskiñ, loskañ [ˈles.kɪ̃] = to burn, calcine, irriate losk = burnt, burning loskadur [los.ˈkɑː.dyr] = burning loskus = burning

Etymology: from the Proto-Indo-European *lh₂p-sḱéti, from *leh₂p- (to light, shine), or from PIE *luk-s-ko-s, from *lewk- (bright, to shine, to see) [source]. Words from the same roots include lamp in English, λάμπω (lámpo – to shine) in Greek, and lāpa (torch) in Latvian.

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, Dictionnaires bilingues de Francis Favereau / Edition Skol Vreizh, TermOfis