[quack] vi [alter. of queck to quack, fr. ME queken, fr. queke, interj., of imit. origin] (14c): to make the characteristic cry of a duck
[2]quack n (1839): a noise made by quacking [3]quack vi [[4]quack] (1628): to act like a quack [4]quack n [short for quacksalver] (1638) 1: charlatan 2
2: a pretender to medical skill -- quack.ish adj [5]quack adj (1653): of, relating to, or characteristic of a quack; esp: pretending to cure diseases