Celtic

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late 16th century: from Latin Celticus (from Celtae ‘Celts’), or from French Celtique (from Celte ‘Breton’).

wiktionary

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From French celtique or Latin celticus. English since the 17th century.

etymonline

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Celtic (adj.)

also Keltic, 1650s, in archaeology and history, "pertaining to the (ancient) Celts," from French Celtique or Latin Celticus "pertaining to the Celts" (see Celt). In reference to the language group including Irish, Gaelic, Welsh, Breton, etc., from 1707. Of modern peoples or their other qualities, by mid-19c. The Boston basketball team was founded 1946. Celtic twilight is from Yeats's name for his collection of adapted Irish folk tales (1893).