From Longman Dictionary of Contemporary Englishknock/throw somebody for a loopknock/throw somebody for a loopAmerican English informalSURPRISED to surprise and upset someone → loop
Examples from the Corpus
knock/throw somebody for a loop• His response really threw me for a loop.• Joanna totally threw me for a loop.• His next question totally knocked me for a loop. He said, "So what makes you think you're good enough to get into law school?" 