According to Wikipedia

According to Wikipedia ...

Schmuck is most often used in English as a pejorative or insult, meaning a stupid or detestable person.
Schmuck entered English as a borrowed word from Yiddish, where it is an obscene term literally meaning a foreskin or head of a penis, and an insult. In his famous cultural lexicon, The Joys of Yiddish, Leo Rosten lists the Yiddish schmuck as related to the Slovene word, šmok, meaning "a fool, an innocent, a gullible dolt".

Ah, so thats what a schmuck is!