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]
- 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.