1774, "of or pertaining to the Gaels" (meaning originally in English the Scottish Highlanders); 1775 as a noun, "language of the Celts of the Scottish Highlands;" earlier Gathelik (1590s), from Gael (Scottish Gaidheal; see Gael) + -ic.
1810, from Scottish Gaelic Gaidheal "member of the Gaelic race" (Irish, Scottish, Manx), corresponding to Old Irish Goidhel (compare Latin Gallus under Gallic, also see Galatians). The native name in both Ireland and Scotland; owing to the influence of Scottish writers Gael was used in English at first exclusively of Highland Scots.
Middle English -ik, -ick, word-forming element making adjectives, "having to do with, having the nature of, being, made of, caused by, similar to," from French -ique and directly from Latin -icus or from cognate Greek -ikos "in the manner of; pertaining to." From PIE adjective suffix *-(i)ko, which also yielded Slavic -isku, adjectival suffix indicating origin, the source of the -sky (Russian -skii) in many surnames. In chemistry, indicating a higher valence than names in -ous (first in benzoic, 1791).
In Middle English and after often spelled -ick, -ike, -ique. Variant forms in -ick (critick, ethick) were common in early Modern English and survived in English dictionaries into early 19c. This spelling was supported by Johnson but opposed by Webster, who prevailed.