throw (or knock) someone for a loop - [informal] surprise or astonish someone; catch someone off guard
to knock /or/ throw sb for a loop - (esp US inf) jdn völlig umhauen (inf)
es haut sie um, oder sie ist außer Tritt geraten.
—witch Mon Jun 21 09:40:28 2004 related discussion:thrown for a loop
:: What is the meaning behind this saying?
: Roughly "stunned me," in either a good or a bad sense. To knock or throw somebody for a loop originally meant to render the person unconscious or (in figurative use) "to make a strong, favorable impression on someone" (Wentworth & Flexner, "Dictionary of American Slang," 1960). "Loop" has been used to mean someone who is stupid or drunk or simply a nobody: "a person with nothing inside" (Dict. Amer. Slang).
: Or simply turned me upside down, or 180 degrees from my original thinking.http://www.phrases.org.uk/bulletin_board/15/m...
I presume that you have already explained that "thrown for a loop" means "bewildered" or "dazzled" or, less frequently, "defeated." The question is why it means that....
The "loop" in a roller coaster or other carnival ride is that portion of the track when the cars travel up in a circular, twisting motion so that at the apex the passengers are traveling upside down, a process also known as "looping the loop."
It is this image of "looping the loop" that underlies "thrown for a loop," the metaphor being that some news or event is sufficiently shocking as to throw the person upside down into the air in a looping motion.
(© 1998 by Evan Morris)http://www.word-detective.com/070698.html
knock for a loop - Also, throw for a loop; knock down or over with a feather; knock sideways. Overcome with surprise or astonishment, as in 'The news of his death knocked me for a loop,' or 'Being fired without any warning threw me for a loop,' or 'Jane was knocked sideways when she found out she won.' The first two of these hyperbolic colloquial usages, dating from the first half of the 1900s, allude to the comic-strip image of a person pushed hard enough to roll over in the shape of a loop. The third hyperbolic term, often put as 'You could have knocked me down with a feather,' intimating that something so light as a feather could knock one down, dates from the early 1800s; the fourth was first recorded in 1925.
(The American Heritage Dictionary of Idioms, © 1997)http://www.answers.com/topic/knock-for-a-loop