be on guard

be on guard

To be very attentive and alert, so as to avoid danger in a particular situation. A possessive pronoun can be used between "on" and "guard." When you're in the big city, you need to be on guard against criminals. I'm on my guard because I know the older guys on the team are going to prank me. I hope she's on her guard. Those trails can be a bit treacherous to climb.

See also: guard, on

Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.