Source : Webster's Revised Unabridged Dictionary (1913)

Quack \Quack\, a.
   Pertaining to or characterized by, boasting and pretension;
   used by quacks; pretending to cure diseases; as, a quack
   medicine; a quack doctor.

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.

Quack \Quack\, n.
   1. The cry of the duck, or a sound in imitation of it; a
      hoarse, quacking noise. --Chaucer.

   2. [Cf. {Quacksalver}.] A boastful pretender to medical
      skill; an empiric; an ignorant practitioner.

   3. Hence, one who boastfully pretends to skill or knowledge
      of any kind not possessed; a charlatan.

            Quacks political; quacks scientific, academical.
                                                  --Carlyle.

Source : WordNet®

quack
     adj : medically unqualified; "a quack doctor" [syn: {quack(a)}]

quack
     n 1: an untrained person who pretends to be a physician and who
          dispenses medical advice
     2: the harsh sound of a duck

quack
     v 1: utter quacking noises; "The ducks quacked"
     2: act as a medical quack or a charlatan