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

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "quack":
   blagueur, bluff, bluffer, cackle, call, carol, caw, charlatan,
   charlatanic, charlatanish, chatter, cheep, chirk, chirp, chirr,
   chirrup, chitter, chuck, clack, cluck, cock-a-doodle-doo, coo,
   counterfeit, counterfeiter, croak, cronk, crow, cuckoo, drum, fake,
   faker, fourflusher, fraud, fraudulent, gabble, gaggle, gobble,
   guggle, honk, hoo, hoot, horse doctor, humbug, impersonator,
   impostor, malingerer, medicaster, medicine man, medicine monger,
   mountebank, peep, phony, pip, pipe, poser, poseur, pretender,
   quackish, quacksalver, quackster, ringer, roll, saltimbanco, scold,
   sham, shammer, simulator, sing, squawk, trill, tweet, twit,
   twitter, warble, whistle



Bouvier's Law Dictionary, Revised 6th Ed (1856):

QUACK. One, who, without sufficient knowledge, study or previous
preparation, and without the diploma of some college or university,
undertakes to practice medicine or surgery, under the pretence that he
possesses secrets in those arts.
     2. He is criminally answerable for his unskillful practice, and also,
civilly to his patient in certain cases. Vide Mala praxis; Physician.