Merriam-Webster's Collegiate® Dictionary, 11th Edition
QUACK
I. intransitive verb
Etymology: alteration of queck to quack, 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 quacking,
III. intransitive verb
Etymology: 4quack
Date: 1628
to act like a quack,
IV. noun
Etymology: short for quacksalver
Date: 1638
charlatan 2, a pretender to medical skill, quackish adjective
V. adjective
Date: 1653
of, relating to, or used by quacks