The Oxford Thesaurus An A-Z Dictionary of Synonyms - quack n.

1) charlatan, impostor, pretender, fake(r) , fraud, Colloq phoney or US also phony: The quack diagnosed Franny as having appendicitis, but it turned out that she was pregnant. --adj. 2) fake, fraudulent, sham, counterfeit, Colloq phoney or US also phony: Are you still going to see that quack doctor - the one who told you to take aspirin to relieve the pain from your ulcer?