(1):
(v. i.) To utter a sound like the cry of a duck.
(2):
(v. i.) To make vain and loud pretensions; to boast.
(3):
(v. i.) To act the part of a quack, or pretender.
(4):
(n.) The cry of the duck, or a sound in imitation of it; a hoarse, quacking noise.
(5):
(n.) A boastful pretender to medical skill; an empiric; an ignorant practitioner.
(6):
(n.) Hence, one who boastfully pretends to skill or knowledge of any kind not possessed; a charlatan.
(7):
(a.) Pertaining to or characterized by, boasting and pretension; used by quacks; pretending to cure diseases; as, a quack medicine; a quack doctor.