quacker (n.)

"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.