always be on (one's) guard

always be on (one's) guard

Always be especially careful or vigilant; always be prepared for danger or surprises. He's got a lot of tricky moves, so you should always be on your guard! She needs to always be on her guard on those trails—they can be really treacherous to climb. When you enter that stage of the game, you need to always be on your guard—bad guys pop up out of nowhere because the lighting is so dim.

See also: always, guard, on

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