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).

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Definitions of Celtic from WordNet
Celtic (n.)
a branch of the Indo-European languages that (judging from inscriptions and place names) was spread widely over Europe in the pre-Christian era;
Synonyms: Celtic language
Celtic (adj.)
relating to or characteristic of the Celts;
Synonyms: Gaelic