'Celtic' originates from Latin, specifically the word 'Celticus', where 'Celt-' referred to the Celts, a group of ancient tribes in Europe.

'Celticus' transformed into the Old French word 'Celtique', and eventually became the modern English word 'Celtic'.

Initially, it referred to the people and languages of the Celts, and this meaning has largely remained the same in modern usage.