What is the X in X-mas?
Old English wærloga "traitor, liar, enemy," from wær "faith, a compact" (cf. Old High German wara "truth," Old Norse varar "solemn promise, vow;" see very; cf. also Varangian) + agent noun related to leogan "to lie" (see lie (v.1)).
Original primary sense seems to have been "oath-breaker;" given special application to the devil (c.1000), but also used of giants and cannibals. Meaning "one in league with the devil" is recorded from c.1300. Ending in -ck and meaning "male equivalent of a witch" (1560s) are from Scottish.