NOUN (2)

1. an untrained person who pretends to be a physician and who dispenses medical advice;

2. the harsh sound of a duck;


VERB (2)

1. utter quacking noises;
- Example: "The ducks quacked"

2. act as a medical quack or a charlatan;


ADJECTIVE (1)

1. medically unqualified;
- Example: "a quack doctor"

The Collaborative International Dictionary of English v.0.48:

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.]
   [1913 Webster]
   1. To utter a sound like the cry of a duck.
      [1913 Webster]

   2. To make vain and loud pretensions; to boast. " To quack of
      universal cures." --Hudibras.
      [1913 Webster]

   3. To act the part of a quack, or pretender.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Quack \Quack\, n.
   1. The cry of the duck, or a sound in imitation of it; a
      hoarse, quacking noise. --Chaucer.
      [1913 Webster]

   2. [Cf. Quacksalver.] A boastful pretender to medical
      skill; an empiric; an ignorant practitioner.
      [1913 Webster]

   3. Hence, one who boastfully pretends to skill or knowledge
      of any kind not possessed; a charlatan.
      [1913 Webster]

            Quacks political; quacks scientific, academical.
                                                  --Carlyle.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

quack
    adj 1: medically unqualified; "a quack doctor"
    n 1: an untrained person who pretends to be a physician and who
         dispenses medical advice
    2: the harsh sound of a duck
    v 1: utter quacking noises; "The ducks quacked"
    2: act as a medical quack or a charlatan