Source : Webster's Revised Unabridged Dictionary (1913)

Quack \Quack\, v. i. [imp. & p. p. {Qvacked}; p. pr. & vb. n.
   {Quacking}.] [Of imitative origin; cf. D. kwaken, G. quacken,
   quaken, Icel. kvaka to twitter.]
   1. To utter a sound like the cry of a duck.

   2. To make vain and loud pretensions; to boast. `` To quack
      of universal cures.'' --Hudibras.

   3. To act the part of a quack, or pretender.