I was curious as to the origin/derivation of the slang expression "knock your socks off". I researched (not extensively, however) and came up with thwo plausible explanations. See below with the links to the sources. I think that the latter is more likely than the former but would be interested in hearing others "chime" in:

In the online source "Word Detective" (02-18-04) it notes that the word “sock” comes from the Latin word “soccus”, meaning “light shoe or slipper" and that the phrase “knock your socks off” apparently first came into use “in the mid-19th century meaning 'to beat or vanquish someone thoroughly', at first used literally to mean to win in a knock-down fistfight so savage that the loser might expect not to only lose his shoes in the fracas but his socks as well.”
· My opinion: Possible but unlikely the source of modern day usage of this expression since in modern days, it is rarely uttered in threat or to describe violence

In the online source "The Answer Bag" (01-16-10) the source claims that the phrase entered modern day usage, i.e. 1940’s and later from the insider’s lexicon in the porn industry. Apparently, in the old “stag” films, the male actors were hard (pardon the pun) to find as they tended to be camera shy and thus often wore disguises or masks and for some reason kept their socks on. In a “quality” or “hot” porn film, the guy wore no socks or disguises, thus it was a film that would “knock your socks off”.
  • My opinion: Much more likely to be the derivation of the modern usage. However, the porn association has been lost with the advent of the video various video formats and the proliferation of the availability of non-camera-shy male actors.