Merriam-Webster Collegiate Dictionary - quack

 I. intransitive verb  Etymology: alteration of queck to ~, from Middle English queken, from queke, interjection, of imitative origin  Date: 14th century to make the characteristic cry of a duck  II. noun  Date: 1798 a noise made by ~ing  III. intransitive verb  Etymology: 4~  Date: 1628 to act like a ~  IV. noun  Etymology: short for ~salver  Date: 1638  1. charlatan 2  2. a pretender to medical skill  • ~ish adjective  V. adjective  Date: 1653 of, relating to, or used by ~s ~ cancer cures