quack v.

"to make a duck sound; utter a harsh, flat, croaking cry," 1610s, earlier quake (late 14c.), variant of quelke (early 14c.), all of echoic origin (compare Middle Dutch quacken, Old Church Slavonic kvakati, Latin coaxare "to croak," Greek koax "the croaking of frogs," Hittite akuwakuwash "frog").

In the same line of Chaucer, various early editions have it as quake, quakke, quak, quat. Frequentative form quackle is attested from 1560s. Middle English on the quakke (14c.) meant "hoarse, croaking." The sense of "talk or advertise noisily and ostentatiously" (1650s) might show influence of quack (n.1). Related: Quacked; quacking.

"medical charlatan, impudent and fraudulent pretender to medical skill," 1630s, short for quacksalver (1570s), from obsolete Dutch quacksalver (modern kwakzalver), literally "hawker of salve," from Middle Dutch quacken "to brag, boast," literally "to croak" (see quack (v.)) + salf "salve," salven "to rub with ointment" (see salve (n.)). As an adjective from 1650s.

The oldest attested form of this quack in English is as a verb, "to play the quack" (1620s). The Dutch word also is the source of German Quacksalber, Danish kvaksalver, Swedish kvacksalvare.

A quack is, by derivation, one who talks much without wisdom, and, specifically, talks of his own power to heal ; hence, any ignorant pretender to medical knowledge or skill. Empiric is a more elevated term for one who goes by mere experience in the trial of remedies, and is without knowledge of the medical sciences or of the clinical observations and opinions of others; hence, an incompetent, self-confident practitioner. A mountebank is generally a quack, but may be a pretender in any line. Charlatan (literally 'chatterer') is primarily applied, not to a person belonging to any particular profession or occupation, but to a pretentious cheat of any sort. [Century Dictionary, 1897]

 Also "one who pretends to knowledge of any kind" (1630s).

"duck sound; a harsh, croaking cry," 1839, from quack (v.). Earlier it meant "hoarseness, croaking" (late 14c.). Quack-quack as a nursery name for a duck is attested by 1865 (quack-quack-quack in that sense is by 1825).