Sentence:  "Polly" (to security guard, referring to Dr. Feingarten): Are you going to let that shyster in there? / "Dr. Feingarten": I could sue you, Polly. A shyster is a disreputable lawyer. I'm a <WSD>quack</WSD>. 
Meaning of the word marked with <WSD> tags:  (doctor) A fraudulent healer or incompetent professional; especially, a doctor of medicine who makes false diagnoses or inappropriate treatment; an impostor who claims to have qualifications to practice medicine. (from "c." 1630)