quack (v.)

"to make a duck sound," 1610s, earlier quake (1520s), variant of quelke (early 14c.), of echoic origin (compare Middle Dutch quacken, Old Church Slavonic kvakati, Latin coaxare "to croak," Greek koax "the croaking of frogs," Hittite akuwakuwash "frog"). Middle English on the quakke (14c.) meant "hoarse, croaking." Related: Quacked; quacking.

quack (n.1)

"medical charlatan," 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 (v.)). As an adjective from 1650s. The oldest attested form of the word in this sense 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.

quack (n.2)

duck sound, 1839, from quack (v.).

Others are reading

Definitions of quack from WordNet
quack (v.)
utter quacking noises;
The ducks quacked
quack (v.)
act as a medical quack or a charlatan;
quack (n.)
an untrained person who pretends to be a physician and who dispenses medical advice;
quack (n.)
the harsh sound of a duck;
quack (adj.)
medically unqualified;
a quack doctor