catch off balance
catch off balance {v. phr.}
To confront someone with physical force or with a statement or question he or she is not prepared to answer or deal with; to exploit the disadvantage of another.
The smaller wrestler caught his opponent off balance and managed to throw him on the float in spite of his greater weight and strength.
Your question has caught me off balance; please give me some time to think about your problem.
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Source: A Dictionary of American Idioms