quack \quack\, v. i. imp.  p. p. qvacked; p. pr.  vb. n.
   quacking. of imitative origin; cf. d. kwaken, g. quacken,
   quaken, icel. kvaka to twitter.
   1913 webster
   1. to utter a sound like the cry of a duck.
      1913 webster

   2. to make vain and loud pretensions; to boast. " to quack of
      universal cures." --hudibras.
      1913 webster

   3. to act the part of a quack, or pretender.
      1913 webster
see also:
qvacked quacking