Warn

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Old English war(e)nian, wearnian, from a West Germanic base meaning ‘be cautious’; compare with ware2.

wiktionary

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From Middle English warnen, warnien(“to warn; admonish”), from Old English warnian(“to take heed; warn”), from Proto-Germanic *warnōną(“to warn; take heed”), from Proto-Indo-European *wer-(“to be aware; give heed”). Cognate with Dutch waarnen(obsolete), German Low German warnen, German warnen, Swedish varna, Icelandic varna.

etymonline

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warn (v.)

Old English warnian "to give notice of impending danger," also intransitive, "to take heed," from Proto-Germanic *warōnan (source also of Old Norse varna "to admonish," Old High German warnon "to take heed," German warnen "to warn"), from PIE root *wer- (4) "to cover." Related: Warned; warning.