Quack

Many people wrongly believe that 'quack' is slang for doctor. In fact, quack is not slang at all and is a term for a medical charlatan. Quack in this sense has been in the language since the early 1600s. Therefore, strictly speaking, it would be highly insulting to refer to a qualified doctor as a quack. Quack is an abbreviated form of ‘quacksalver’, which is an even earlier word, dating from the 1500s, meaning a charlatan that peddles salves or cures. The etymology of quack is Middle Dutch 'quacken' meaning to brag or boast. In the context of medicinal practice, it means bragging or boasting about fictitious medical qualifications. Quack, as in the sound that a duck makes, also dates from the early 1600s but it is purely imitative or echoic of the sound that a duck makes.