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.
We heard Mr.Hodson's whip clacking on the ahoulders
of the poor little wretches. --Thackeray.
2. To utter words rapidly and continually, or with
abruptness; to let the tongue run.