From Scottish Gaelic 'carn' (heap of stones), from Proto-Celtic *karno- — among humanity's oldest architectural forms.
A mound of rough stones built as a memorial or landmark.
From Scottish Gaelic càrn (heap of stones, rocky hill), from Old Irish carn (cairn, heap), from Proto-Celtic *karno- (heap of stones, stony place), possibly from PIE *kar- (hard, stone). The Proto-Indo-European root *kar- (hard) may connect cairn to a broad family of words for stone and hardness, though the Celtic origin is certain and the deeper etymology debated. In Gaelic and Irish, càrn / carn designated both natural rocky hills and deliberate stone piles used as trail markers, burial monuments
The word is one of relatively few Celtic borrowings in English.