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.

Source: Webster's Revised Unabridged Dictionary (1913)
 

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.]

1. To utter a sound like the cry of a duck.

2. To make vain and loud pretensions; to boast. `` To quack of universal cures.'' --Hudibras.

3. To act the part of a quack, or pretender.

Source: Webster's Revised Unabridged Dictionary (1913)
 

Quack \Quack\, n.

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

2. [Cf. Quacksalver.]

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.

Quacks political; quacks scientific, academical. --Carlyle.

Source: Webster's Revised Unabridged Dictionary (1913)