Meaning

noun (Yiddish) a big shot who knows it and acts that way; a boastful immoderate person

noun a person who knocks (as seeking to gain admittance)

noun one who disparages or belittles the worth of something

originFrom the verb knock + -er, an agentive suffix. The slang term for breasts is attested since the 1940s. For fuller etymology see knock.

Etymology from Wiktionary, CC BY-SA 3.0