schmuck (n.)

also shmuck, "contemptible person," 1892, from East Yiddish shmok, literally "penis," probably from Old Polish smok "grass snake, dragon," and likely not the same word as German Schmuck "jewelry, adornments," which is related to Low German smuck "supple, tidy, trim, elegant," and to Old Norse smjuga "slip, step through" (see smock).

In Jewish homes, the word was "regarded as so vulgar as to be taboo" [Leo Rosten, "The Joys of Yiddish," 1968] and Lenny Bruce wrote that saying it on stage got him arrested on the West Coast "by a Yiddish undercover agent who had been placed in the club several nights running to determine if my use of Yiddish terms was a cover for profanity." Euphemized as schmoe, which was the source of Al Capp's cartoon strip creature the shmoo.

"[A]dditional associative effects from German schmuck 'jewels, decoration' cannot be excluded (cross-linguistically commonplace slang: cf. Eng. 'family jewels')" [Mark R.V. Southern, "Contagious Couplings: Transmission of Expressives in Yiddish Echo Phrases," 2005]. But the English phrase refers to the testicles and is a play on words, the "family" element being the essential ones. Words for "decoration" seem not to be among the productive sources of European "penis" slang terms.