"be on one's guard," c. 1200, probably a contraction of be ware "be wary, be careful," from Middle English ware (adj.), from Old English wær "prudent, aware, alert, wary," from Proto-Germanic *waraz, from PIE root *wer- (3) "perceive, watch out for." Old English had the compound bewarian "to defend," which perhaps contributed to the word. Compare begone.
