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Know Your Phrase - The Meanings For Common Phrases and Idioms > K-Letter Sayings
The origins of this idiom.
The meaning of this phrase.
K
Phrases and Idioms
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To be taken by surprise.
* When Hannah gets home from work later today, I have something planned that is going to really knock her socks off.
Note: The origins for most popular idioms cannot be said with a certainty. What's provided are theories that may be plausible to how a phrase originated, but not necessarily so.

In addition, quotes that contain a particular phrase may be taken from old newspapers, poems, or books that were written centuries ago, but this by no means confirms that the phrase originates from said newspapers, poems, or books. In all likelihood, if an expression is being used in a newspaper, it's probably already a well known saying and is from an older time.
The phrase looks to have gotten popular around the mid-19th century, and while
today the phrase is typically expressed to describe someone who is taken by
surprise, back then it looks to have been mostly used in the sense of 'to defeat
someone or something thoroughly, or completely.'

For example, in the Logansport Democractic Pharos newspaper, Janurary 1856, common flu symptoms are said to be defeated soundly by a particular remedy:

"The promptness and certainty with which the Ague King's American remedy for Chills and Fever,
knocks the socks off that disease."