Quack

Quack or quacksalver is a derogatory term for a person within the field of medicine whose practice is suspect, unprofessional, or bad. Such individuals have either swindled people of money or, through their practice, have caused harm. There are also quacks who believe in what they are doing and are operating under pure motives, but they are nevertheless wrong.

Quack is also an onomatopoeia for the noise that a duck makes.