The Collaborative International Dictionary of English v.0.48:

Clack \Clack\, v. i. [imp. & p. p. Clacked; p. pr. & vb. n.
   Clacking.] [Prob. of imitative origin; cf. F.claquer to
   clap, crack, D. klakken, MHG. klac crack, Ir. clagaim I make
   a noise, ring. Cf. Clack, n., Clatter, Click.]
   1. To make a sudden, sharp noise, or a succesion of such
      noises, as by striking an object, or by collision of
      parts; to rattle; to click.
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            We heard Mr.Hodson's whip clacking on the ahoulders
            of the poor little wretches.          --Thackeray.
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   2. To utter words rapidly and continually, or with
      abruptness; to let the tongue run.
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