An Etymological Dictionary of the German Language/warnen

by Friedrich Kluge

warnen, vb., ‘to warn, admonish,’ from MidHG. warnen, ‘to watch over, protect,’ OHG. warnen (węrnen), ‘to deny, refuse, decline.’ Corresponding to OSax. węrnian, ‘to decline, withhold.’ AS. wyrnan, E. warn, OIc. varna, ‘to refuse.’ On account of the meaning the connection with OHG. warnôn (see wahren) is dubious; undoubted cognates have not yet been found.