quack

v. i.

  1. To utter a sound like the cry of a duck.
  2. To make vain and loud pretensions; to boast.
  3. To act the part of a quack, or pretender.

n.

  1. The cry of the duck, or a sound in imitation of it; a hoarse, quacking noise.
  2. 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.

a.

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