The phrase knock or blow someone’s socks off was initially used in southern part of US in the 1940s, where this phrase referred as defeating someone in the fight. Later, the following meanings were also assigned to it (mostly positive meanings)
Doing something which was never done before / impressing someone/ performing something outclass.
Example
Thushar’s performance at the concert last night just knocked my socks off
This exciting news knocked the socks off of everyone in the school.