(kwak) v.i.
To utter a harsh, croaking cry, as a duck.
—noun The sound made by a duck, or a similar croaking noise. [Imit].

(kwak) noun
1. A pretender to medical knowledge or skill.
2. A charlatan.
—adjective Of or pertaining to quacks or quckery; ignorantly or falsely pretending to cure.
—v.i. To play the quack. [Short for QUACKSALVER]
—quack′ish adjective
—quack′ish·ly adverb Synonyms (noun): charlatan, empiric, humbug, impostor, mountebank. Antonyms: adept, expert, master.