catch one off

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 one off

(one's) guard Go to catch someone off guard.
See also: catch, off, one
References in periodicals archive ?
And then there is his androgynous physical appearance, which can easily catch one off guard.