What is the X in X-mas?
1530s, as an adjective, "joyous, merry," from Middle Dutch vrolyc (adj.) "happy," from vro- "merry, glad," + lyc "like." Cognate with German fröhlich "happy." The stem is cognate with Old Norse frar "swift," Middle English frow "hasty," from PIE *preu- (see frog (n.1)), giving the whole an etymological sense akin to "jumping for joy." The verb is first attested 1580s. Related: Frolicked; frolicking. As a noun, from 1610s.