Schmuck, also spelled shmuck, is a Yiddish word meaning a contemptible, low person.
The origin of schmuck is the East Yiddish word shmok, which literally means penis, and is probably derived from the old Polish word “smok” for grass snake or dragon.
In current usage schmuck is used more in the sense of a fool or asshole rather than penis.
Incidentally, schmuck is also the German word for jewelry, but this is unlikely to share etymology with the schmuck explained here.
“Hey, stop being such a schmuck and start standing up for yourself!”.