Middle English, from Old English warnian; akin to Old High German warnōn to take heed, Old English wær careful, aware — more at wary
Date:
before 12th century
transitive verb1 a: to give notice to beforehand especially of danger or evilb: to give admonishing advice to :counselc: to call to one's attention :inform2: to order to go or stay away —often used with offintransitive verb: to give a warning