From Longman Dictionary of Contemporary Englishcatch/throw somebody off guardcatch/throw somebody off guardREADYto surprise someone by doing something that they are not ready to deal with Senator O'Hare was caught off guard by the question. → guard
Examples from the Corpus
catch/throw somebody off guard• Could the upper management of a leading firm like Merrill Lynch be caught so entirely off guard?• The president faces issues that can catch him off guard and undermine his authority.• Penelope flinched, angry at her thoughts, and at the girl who had caught her off guard.• I said it suddenly like that, just blurted it out, and I guess it caught him off guard.• The question caught Firebug off guard.• She really caught me off guard with her comments.• This caught me completely off guard.• The words caught him off guard. 