Old English wærloga "traitor, liar, enemy, devil," from wær "faith, fidelity; a compact, agreement, covenant," from Proto-Germanic *wera- (source also of Old High German wara "truth," Old Norse varar "solemn promise, vow"), from PIE root *were-o- "true, trustworthy." Second element is an agent noun related to leogan "to lie" (see lie (v.1); and compare Old English wordloga "deceiver, liar").
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 (1680s) and meaning "male equivalent of a witch" (1560s) are from Scottish. Related: Warlockery.