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
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

catch one off

(one's) guard Go to catch someone off guard.
See also: catch, off, one
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
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References in periodicals archive ?
I am not going to get in the middle and catch one off them!
And then there is his androgynous physical appearance, which can easily catch one off guard.