warlock

From Middle English warloghe, warlowe, warloȝe, from Old English wǣrloga (“traitor, deceiver”, literally “truce-breaker”), from Proto-West Germanic *wārulogō (“liar”), equivalent to Old English wǣr (“covenant, truce, pact, promise”) (from Proto-Indo-European *weh₁- (“true”); whence also Latin vērus) + loga (“liar”), from Proto-Germanic *lugô, related to Old English lēogan (whence English lie). The hard -ck ending originated in Scottish and Northern English, like the sense "male magic-user" (from the notion that such men were in league with the Devil and had thus broken their baptismal vows / betrayed Christianity). Cognate with Old Saxon wārlogo (“liar, unfaithful or insidious one”).
A few writers alternatively propose a derivation from Old Norse varðlokkur (“incantations, charms”, literally “ward songs”), but as the OED notes, this is implausible due to the extreme rarity of the Norse word, the semantic difference, and because forms without hard -k, which are consistent with the Old English etymology (“traitor”), are attested earlier than forms with -k, and forms with -ð- are not attested.