vi to utter a sound like the cry of a duck.
2. quack ·vi to act the part of a quack, or pretender.
3. quack ·vi to make vain and loud pretensions; to boast.
4. quack ·noun a boastful pretender to medical skill; an empiric; an ignorant practitioner.
5. quack ·noun the cry of the duck, or a sound in imitation of it; a hoarse, quacking noise.
6. quack ·noun hence, one who boastfully pretends to skill or knowledge of any kind not possessed; a charlatan.
7. quack ·adj pertaining to or characterized by, boasting and pretension; used by quacks; pretending to cure diseases; as, a quack medicine; a quack doctor.