be on guard(redirected from are on their 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.
be on (your) guard
to be careful to avoid being tricked or getting into a dangerous situation I feel I have to be on my guard with her because she's always trying to get information out of me. (often + against ) Shop assistants must always be on guard against shoplifters.