Q From Barry Bloom, California: I am wondering about thrown for a loop, which means to be greatly surprised. Can you tell me where it comes from?
A All sorts of suggestions have been put forward — with greatly varying levels of certitude — about the image behind this American expression. The most popular ones include an aircraft looping the loop, a person being physically knocked head over heels, or a calf brought down by a lariat looped around a leg. In an entry written many years ago, the Oxford English Dictionary suggests that it may derive from passengers looping the loop in an early roller coaster. (It charmingly calls it by the obsolete term centrifugal railway, this name having been given to several looping rides in the early nineteenth century, originally in France but then from the 1840s in the UK and the USA.)
Part of the problem is that there are actually two forms of the expression, knock somebody for a loop and throw somebody for a loop, and various meanings for both. You might be saying that you have surprised, astonished, shocked or confused a person, caught them off guard, or made a strong impression on them. You might even be saying that that they’ve actually been knocked down.
The written evidence — which is all we’ve got to go on — dates from the early 1920s. There’s no doubt that the first form was knocked for a loop and was a sporting term, especially in boxing:
Round after round, the fight goes on with continued reports of heavy punching, suddenly followed by a loud roar from the crowd. Father: “Listen! Somebody got knocked for a loop sure as guns!”
The Wireless Age, Aug. 1921. This is part of its report into the historic broadcast of the Dempsey-Carpentier heavyweight championship prize fight by the WJY station of New York on 2 Jul. 1921.
Casey Is Knocked For a Loop Early By Everett Boxer
A headline in the Oakland Tribune, 22 Jul. 1922.
This suggests that the original idea may have been a punch that was heavy enough to lay out an opponent by making him fall backwards and roll over. It may always have been a metaphor; certainly it’s already become so even in these early appearances — the report under the headline in the Oakland Tribune refers to an easy win, not to a specific blow. Another early use shows that it had already been in the language long enough to gather the sense of something that surprises or astonishes:
“Cut the bread,” she commanded, “and I’ll make some bacon sandwiches that will knock you for a loop.” He wheeled toward the bread-box and reached for a loaf.
The Last Mile, by Frank A McAlister, 1922.
Throwing somebody for a loop, on the other hand, doesn’t appear for about another decade and seems always to have had the idea behind it that you mention — primarily surprise. Did its early users want a more forceful saying than the baseball-originated throw someone a curve? Or did they have judo in mind? Or had they just created a variation on knock for a loop without thinking about it? I rather suspect the last of these. But trying to get inside the minds of casual creators of idioms 80 years after the event is always going to be difficult.