warn (v.) Look up warn at Dictionary.com
Old English warnian "to give notice of impending danger," also intransitive, "to take heed," from Proto-Germanic *warnon (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.