Etymology Dictionary

QUACK (1),  to make a noise like a duck.  (E.)   An imitative word.  'The goos, the duk, and the cuckow also So cried "keke! keke!" "cuckow!" "queke, queke!" hye;' Chaucer, Parl. of Foules, 499.   Here the cry keke! keke! is assigne to the cackling goose, and queke! queke! to the quacking duck.  In Ch. C. T. 4150, the dat. case quakke is used to mean 'hoarseness.' + Du. kwaken, to croak, quack, chat. + G. quaken, to quack, croak. + Icel. kvaka, to twitter. + Dan. qvække, to croak, quack, cackle.   Cf. Lat. coaxare, to croak, Gk. κωάξ, a croaking; Lithuan. kwakëti, to croak; kwaksëti, to cackle.   β. A mere variant of the base KAK seen in Cackle, q.v.  Der. quack (2), q.v.   Also quail (2), q.v. 

QUACK (2),  to cry up pretended nostrums.  (E.)  Merely a particular use of Quack (1).  It means to chatter about, cackle or prate of, hence, to sing the praises of a nostrum, to pretend to medical skill.   'To quack off universal cures;' Butler, Hudibras, pt. iii. c. I. l. 330.   Der. quack-salver, Blount's Gloss., ed. 1674, i.e. a quack who puffs up his salves or ointments, borrowed from Du. kwak-zalver, a quack, charlatan, cf. Du. kvakzalven, to quack, puff up salves (see Salve); quack-doctor, a later word which took the place of quack-salver, Pope, note to Dunciad, iii. 192.   Hence also quack = quack-doctor; quack-er-y.