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general::
Phrase(s): catch one off (one’s) guard
Go to catch someone off guard., Phrase(s): catch someone off guard [and] catch one off (one’s) guard
Fig. to catch a person at a time of carelessness. • Tom caught Ann off guard and frightened her. • She caught me off my guard, and my hesitation told her I was lying.
McGrawhill's American Idioms And Phrasal Verbs