Real Dictionary

quack
If you call someone a quack or a quack doctor, you mean that they claim to be skilled in medicine but are not.
Quack remedies or quack cures are medical treatments that you think are unlikely to work because they are not scientific.

quack in American English 1
an untrained person who practices medicine fraudulently
characterized by pretentious claims with little or no foundation
to engage in quackery
intransitive verb: to utter the cry of a duck or a sound resembling it
noun: the harsh, throaty cry of a duck or any similar sound

quack in American English 2
the sound made by a duck, or any sound like it
to utter the characteristic sound or cry of a duck, or a sound like it
adjective: being a quack
noun: a fraudulent or ignorant pretender to medical skill
transitive verb: to treat in the manner of a quack

quack in British English 1
(of a duck) to utter a harsh guttural sound
noun: the harsh guttural sound made by a duck

quack in British English 2
noun: an unqualified person who claims medical knowledge or other skills
verb: to act in the manner of a quack