From The Collaborative International Dictionary of English v.0.48:

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.]
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   1. To utter a sound like the cry of a duck.
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   2. To make vain and loud pretensions; to boast. " To quack of
      universal cures." --Hudibras.
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   3. To act the part of a quack, or pretender.
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