Source : Webster's Revised Unabridged Dictionary (1913)

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.

            We heard Mr.Hodson's whip clacking on the ahoulders
            of the poor little wretches.          --Thackeray.