catch someone off guard
catch (one) off guard
To engage, take advantage of, or seize upon one when one is not especially careful, vigilant, or prepared for danger or surprises. He's got a lot of tricky moves, so make sure he doesn't catch you off guard! The soldiers moved in quickly, hoping to catch the enemy troops off guard. I think I caught him off guard when I told him I was resigning.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
catch someone off guardor
take someone off guard
COMMON If something catches you off guard or takes you off guard, it happens unexpectedly and you are not prepared to respond to it. Note: Your guard is the position that you get in when you are ready to defend yourself in sports such as boxing or fencing. I didn't know how to answer him. The question caught me off guard. I wasn't expecting him to pass the ball to me and it took me off guard.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012