We often hear the subject expression and "knock someone's socks off" and it is not difficult to understand what it means.
The Link shows what it means and how it originated:
- Overwhelm, bedazzle, or amaze someone, as in The young pianist knocked the socks off of the judges, or That display will knock their
socks off. [Slang; mid-1800s]
or
- Also, knock the spots off. Surpass or outdo completely, defeat. For example, These large chains have been knocking the socks off the small
independent grocers, or Our team knocked the spots off them. The spots
most likely allude to target practice with playing cards where the
object is to shoot through all the pips, spots, or marks indicating
the suit or numerical value of a playing card, but one authority holds
that they were used in a horse-breeding context and meant "to be in
the vanguard." [Mid-1800s]
But this and other links don't show explanations on why the verb, knock, was used for the socks which cannot be easily taken off by knocking.
Best Answer
I think it comes from its original meaning of 'beat/strike', the idea is that you knock someone to the point that even their socks come off, that is hard, violently, (both physically but mainly figuratively):
Knock:
( Eynonline)
(www.knowyourphrase.com)
(www.quora.com)