Definitions from Wiktionary (quack)

▸ noun:  The sound made by a duck.
▸ verb:  To make a noise like a duck.
▸ verb:  (intransitive) Of a queen bee: to make a high-pitched sound during certain stages of development.
▸ noun:  (derogatory) A fraudulent healer, especially a bombastic peddler in worthless treatments, a doctor who makes false diagnoses for monetary benefit, or an untrained or poorly trained doctor who uses fraudulent credentials to attract patients
▸ noun:  (figuratively, derogatory) Any similar charlatan or incompetent professional.
▸ noun:  (humorous slang, mildly derogatory) Any doctor.
▸ verb:  To practice or commit quackery (fraudulent medicine).
▸ verb:  (obsolete) To make vain and loud pretensions.
▸ adjective:  Falsely presented as having medicinal powers.

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