Definition of quack:

part of speech: verb

To cry like a duck.

part of speech: verb

To act or practise as a quack.

part of speech: adjective

Falsely pretending to cure diseases; pert. to or tainted with quackery.

part of speech: noun

The cry of a duck.

part of speech: noun

An ignorant pretender to medicine; a boastful pretender to skill or knowledge not possessed; an empiric.