1
general::
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.
American Heritage Idioms
2
general::
Phrase(s): knock someone for a loop
1. Fig. to strike someone hard. • You really knocked me for a loop. I hope that was an accident. • I was really knocked for a loop by the falling branch. 2. and throw someone for a loop Fig. to confuse or shock someone. (This is more severe and upsetting than throw someone a curve.) • When Bill heard the news, it threw him for a loop. • The manager knocked Bob for a loop by firing him on the spot.
McGrawhill's American Idioms And Phrasal Verbs