A Yiddish word that means literally, penis; as in don't touch your schmuck! However, it is quite useful to use it in a personal context such as "John Doenstein is a Schmuck." In this connotation schmuck means a clumsy person or an oaf.
It has been argued that the meaning of this word is derived from the German word Schmucksachen (jewelry). This is in fact not true! The word shmuck was originally spoken as smok, the Polish word for serpent or tail. When this word is spoken with a Yiddish/Hebrew accent it comes out sounding closer to "schmuck."
Don't argue with the OED!