catch somebody 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

catch somebody off (their) ˈguard

happen when somebody is not prepared: The question caught him off his guard and he couldn’t answer.Businesses were caught off guard by the sudden rise in interest rates.
See also: catch, guard, off, somebody