Noah Webster
Quack
QUACK, verb intransitive
1. To cry like a duck or goose.
2. To boast; to bounce; to talk noisily and ostentatiously; as, pretenders to medical skill quack of their cures.
QUACK, noun [from the verb.]
1. A boaster; one who pretends to skill or knowledge which he does not possess.
2. A boastful pretender to medical skill which he does not possess; an empiric; an ignorant practitioner.