Noun

1. The sound made by a duck.

   Did you hear that duck make a quack?

Verb

1. To make a noise like a duck.

   The more breadcrumbs I threw on the ground, the more they quacked.

   Do you hear the ducks quack?

Etymology 2

Noun

1. A fraudulent healer or incompetent doctor of medicine, an impostor who claims to have qualifications to practice medicine.

   That doctor is nothing but a lousy quack!

2. A charlatan.
3. A doctor.

Quotations

• 1662: Rump: or an Exact Collection of the Choycest Poems and Songs Relating to Late Times, Vol. II, by ‘the most Eminent Wits’

  Tis hard to say, how much these Arse-wormes do urge us, We now need no Quack but these Jacks for to purge us, [...]

• 1720: William Derham, Physico-theology

  After ſome Months, the Quack gets privately to Town, [...]

• 1843, Thomas Carlyle, Past and Present, book 2, ch. 8, The Electon

  ‘if we are ourselves valets, there shall ‘exist no hero for us; we shall not know the hero when we see him;’ - we shall take the quack for a hero; and cry, audibly through all ballot-boxes and machinery whatsoever, Thou art he; be thou King over us!

Adjective

1. falsely presented as having medicinal powers.

   Don't get your hopes up; that's quack medicine!