on your guard
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on (one's) guard
Especially careful or vigilant; prepared for danger or surprises. He's got a lot of tricky moves, so be on your guard! Those trails can be a bit treacherous to climb. I just hope she's on her guard.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
on your guardor
COMMON If you are on your guard or on guard, you are prepared to protect yourself against trouble. Note: Your guard is the position that you get in when you are ready to defend yourself in sports such as boxing or fencing. When Hilton came, he would have to be on his guard each second, for the man was dangerous. A journalist asked a question about his private life and instantly, the footballer was on guard. Compare with off your guard.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012