The Collaborative International Dictionary of English v.0.48:

Croak \Croak\ (kr[=o]k), v. i. [imp. & p. p. Croaked.
   (kr[=o]kt); p. pr. & vb. n. Croaking.] [From the primitive
   of AS. cracettan to croak as a raven; akin to G. kr[aum]chzen
   to croak, and to E. creak, crake.]
   1. To make a low, hoarse noise in the throat, as a frog, a
      raven, or a crow; hence, to make any hoarse, dismal sound.
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            Loud thunder to its bottom shook the bog,
            And the hoarse nation croaked.        --Pope.
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   2. To complain; especially, to grumble; to forebode evil; to
      utter complaints or forebodings habitually.
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            Marat . . . croaks with reasonableness. --Carlyle.
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