Woman: “You're awfully shy for a lawyer.”
Groucho: “You bet I'm shy...I'm a shyster lawyer!"
- Monkey Business (1931)
The definition of “shyster”, according to the Free Online Dictionary, is:
a lawyer or politician, who uses discreditable or unethical methods
An unscrupulous lawyer (note that the deﬁnition presumes the existence of scrupulous ones) . . .
The term does not come from—as suggested in various dictionaries—the surname Scheuster, supposedly a lawyer noted for shyster-like practices; from the name of the Shakespearean character, Shylock; . . . or from any of the various meanings of shy (e.g., to be shy of money). Rather . . . shyster evolved from the underworld use of shiser, a worthless fellow, which derived in turn from the German scheisse, excrement, via scheisser, an incompetent person (speciﬁcally, one who cannot control his bodily functions) . . .
Hugh Rawson (1991)
A Dictionary of Invective