Gorm's S-Word Awards: Definitions.
Schmuck.
A schmuck is a stupid, obnoxious, contemptible person. Schmuck is literally Yiddish for penis; a schmuck, basically, is a prick.
Schlemiel.
A schlemiel is an unlucky bungler, a dolt. Probably comes from Shelumiel, son of Zurishaddai (Numbers 7.36), a prince of the tribe of Simeon. According to the Talmud, Shelumiel was killed while committing adultery.
Schlimazel.
An unfortunate person for whom everything goes wrong. Slimp, wrong or crooked, + mazl, luck.
Somebody once described the difference between a schlemiel and a schlimazel thus: A schlemiel is a person who goes through life spilling soup. A schlimazel is the person he spills it on.