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*on (one's) guard (against someone or something)
alert against someone or something. (*Typically: be ~; keep ~; remain ~; stay ~.) Try to stay on guard against pickpockets. I am always on my guard. Be on guard when you go into the city.
on your guard
to be careful and aware because a situation might be dangerous I resent this attitude that you can't trust anybody, that you always have to be on your guard.
on (one's) guard
Alert and watchful; cautious.