catch somebody off balance

Từ điển Oxford Advanced Learner 8th

(catch/throw sb) off ˈbalance idiom
1. to make sb/sth unsteady and in danger of falling
•I was thrown off balance by the sudden gust of wind.
2. to make sb surprised and no longer calm
•The senator was clearly caught off balance by the unexpected question.
Main entry: ↑balance_idiom