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.
See also: catch, guard, off
Farlex Dictionary of Idioms.

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, someone
Collins COBUILD Idioms Dictionary, 3rd ed.
See also:
  • 1FTR
  • drive (one) out of office
  • force (one) out of office
  • force out of office
  • give (one) (one's) head
  • give head
  • give somebody their head
  • give someone their head
  • deep water
  • cooking for one
References in periodicals archive
Catch someone off guard and show you mean business.
stars
"By then I decided to take it out hard and see if I could catch someone off guard. I ended up holding it for a little more than two miles, but then I died down a bit.
Costin's quest is to join state's best in long run
It wasn't like she was wandering through Mothercare waiting to catch someone off guard.
How dare they help orphans