throw someone for a loop

throw (one) for a loop

To shock, surprise, astonish, or bewilder one. The end of that trick always throws the audience for a loop. I love watching their faces as they desperately try to figure it out. It really threw Stu for a loop when Olivia announced she was leaving the company.
See also: loop, throw
Farlex Dictionary of Idioms.

throw someone for a loop

or

knock someone for a loop

AMERICAN, INFORMAL
If someone or something throws you for a loop or knocks you for a loop, they shock you or surprise you very much. If Kravis's offer had thrown him for a loop, Johnson wasn't letting it show. She was wearing a top hat, the sight of which knocked Jamie for a loop.
See also: loop, someone, throw
Collins COBUILD Idioms Dictionary, 3rd ed.

throw (or knock) someone for a loop

surprise or astonish someone; catch someone off guard. North American
See also: loop, someone, throw
Farlex Partner Idioms Dictionary

throw someone for a loop

tv. to confuse or surprise someone. Don’t let this question throw you for a loop.
See also: loop, someone, throw
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions
See also:
  • throw (one) for a loop
  • throw (one) for a loss
  • throw for a loss
  • knock (one) for a loop
  • knock for a loop
  • knock/throw somebody for a loop
  • knock over with a feather
  • knock (someone) over with a feather
  • you could have knocked me over with a feather
  • knock down with a feather