I was just saying this today and I realized I have no idea where it comes from. What is the origin of "throw (someone) for a loop"?
Some Google searches show that I've been using it correctly and that it generally means to astonish or confuse someone. I've heard it used in this context: "Wow I wasn't expecting that, it threw me for a loop!"
Best Answer
It may come from the earlier "knocked for a loop", in boxing meaning to be hit in the head causing confusion.
Google Ngrams shows "knocked for a loop" appearing around 1918, and "thrown for a loop" around 1945.
The Word Detective also places its origin at about 1920:
Other sources disagree with the OED theory, preferring to link it to "loopy", which first appeared in the 1820's. The first roller coaster with an inverted loop was built in the 1950s.