Learn English – n idiom for people who boast too much

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:

• Old English cnocian (West Saxon cnucian), "to pound, beat; knock (on a door)," likely of imitative origin. Meaning "deprecate, put down" is from 1892. , Knock-kneed first attested 1774. Knock-down, drag-out is from 1827.

( Eynonline)

• 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."

(www.knowyourphrase.com)

• To knock somebody's socks off off means to surprise somebody by showing or providing them with something really impressive. The phrase was originally documented in the American South in the 1940s, where the phrase referred to beating somebody in a fight (similar to "knock his block off"). For this reason, the phrase originally had negative connotations, but began to acquire more positive connotations as the phrase was used more figuratively as a synonym for astonishing or impressing somebody.

• One major factor in popularizing the positive meaning of "knock your socks off" was an ad campaign done by Pepsi-Cola in the mid-1960s to promote Mountain Dew, a regional Southern soft drink purchased by Pepsi-Cola that Pepsi hoped to make popular on a national basis. Mountain Dew originally got its name for a Southern slang term for moonshine whiskey. So Pepsi naturally decided to launch an ad campaign that played up Mountain Dew as a quintessentially Southern soft drink, but one that could be enjoyed by people from all regions of the United States. Since the stereotype was that Southern people liked to go barefoot, the ad campaign encouraged soft drink buyers to "get that barefoot feeling" by buying Mountain Dew, which the ads claimed would "knock your socks off."

(www.quora.com)

frequently used idiom: thin-skinned

Fig. easily upset or hurt; sensitive. (The opposite of thick-skinned. Typically: be ~; become ~; grow ~.) You'll have to handle Mary's mother carefully. She's very thin-skinned.

https://idioms.thefreedictionary.com/thin-skinned