late 13c., "something which strikes with a loud, sharp noise," agent noun from clap (v.). Meaning "tongue of a bell" is from late 14c. Old English had clipur. Meaning "hinged board snapped in front of a camera at the start of filming to synchronize picture and sound" is from 1940.
Clappering.--It is still necessary to warn clergymen and churchwardens against allowing the lazy and pernicious practice of 'clappering,' i.e. tying the bell-rope to the clapper, and pulling it instead of the bell. More bells have been cracked in that way than by all other causes together, and there is not the least excuse for it .... [Sir Edmund Beckett, "A Rudimentary Treatise on Clocks and Watches and Bells," London, 1883]