To make a quick sharp noise, or a succession of sharp sounds, as by striking or cracking; crack; rattle; snap.To utter sounds or words rapidly and continually, or with sharpness and abruptness; let the tongue run or rattle.To cause to make a sharp, short, snapping sound; rattle; clap: as, to clack two pieces of wood together.To speak without thought; rattle out.n. A sharp, repeated, rattling sound; clatter: as, the clack of a mill.n. In a grist-mill: That part of the mill that strikes the hopper, to move or shake it, for discharging its contents.n. A bell that rings when more corn is required to be put in the hopper.n. A valve of a pump.n. A ball-valve connected with the boiler of a locomotive. See ball-ralve and clack-box, 2.n. A kind of small windmill with a clapper, set on the top of a pole to frighten away birds. Also called clack-mill, and formerly clacket.n. Continual talk; prattle; gossip; tattle.To cluck or cackle, as a hen.