Английский Этимологический словарь - quack

- "duck sound," 14c., quelke, of echoic origin. The noun meaning "medical charlatan" is 1638, short for quacksalver (1579), from Du. kwaksalver, lit. "hawker of salve," from M.Du. quacken "to brag, boast," lit. "to quack (like a duck)" + zalf "salve."