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be on guard

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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.

See also: guard, on

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.

See also: guard, on