QUACK
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Definitions
1. someone who gives service but refuses to refund the fee if the service is unsatisfactory. (6903C27 SO)
2. the quack was a man who purveyed quicksilver (English pronunciation quacksilver) hence quack. He appeared at the English county fairs and purveyed bichloride and mercury which often killed people, but it also cured a lot of diseases. Now, a quack is anybody the American Medical Association doesn't like. (6804SM-)
3. is anyone who pretends to be something which he is not, or one who is not able to do what he claims to do, especially if he takes money for this pretense. (HCO Info Ltr 22 Sept 63)