clack / klak/ • v. make or cause to make a sharp sound or series of such sounds as a result of a hard object striking another: [intr.] he heard the sound of her heels clacking across flagstones | [tr.] he clacked the castanets in fine syncopation. ∎  [intr.] archaic chatter loudly: he will sit clacking for hours.• n. a sharp sound or series of sounds made in such a way: the clack of her high heels. ∎ archaic loud chatter: her clack would go all day.DERIVATIVES: clack·er n.

clack chatter XIII; make a clattering noise XVI. prob. — ON. klaka twitter, (of birds) chatter; of imit. orig.; cf. Du. klakken crack, F. claquer.
Hence clack sb. clatter of talk XV; clapping or clacking noise XVI.