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.

Shop assistants must always be on guard against shoplifters. (often + against)

• could do something in their sleep
• could do something standing on head
• could do something standing on your head
• could do something with eyes closed
• could do something with eyes shut
• could do something with one arm tied behind back
• could do something with one arm tied behind their back
• could do something with one hand tied behind back
• could do something with one hand tied behind their back
• could do something with their eyes closed
• could do something with their eyes shut
• could do with
• could do with one arm tied behind back
• could do with one arm tied behind their back
• could do with one hand tied behind back
• could do with one hand tied behind their back
• could do with something
• could hear a pin drop
• could in sleep
• could in their sleep
• couldn't [act/argue/fight] way out of a paper bag
• couldn't [act/argue/fight] your way out of a paper bag
• couldn't act way out of a paper bag
• couldn't act your way out of a paper bag
• couldn't argue way out of a paper bag