Etymology
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warlock (n.)
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.
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*were-o- 
*wērə-o-, Proto-Indo-European root meaning "true, trustworthy."

It forms all or part of: aver; Varangian; veracious; veracity; verdict; veridical; verify; verisimilitude; verism; veritas; verity; very; voir dire; warlock.

It is the hypothetical source of/evidence for its existence is provided by: Latin verus "true;" Old Church Slavonic vera "faith," Russian viera "faith, belief;" Old English wær "a compact," Old Dutch, Old High German war, Dutch waar, German wahr "true;" Welsh gwyr, Old Irish fir "true."
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