catch someone off guard

catch someone off guard

or

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.
See also: catch, guard, off