clapper (n.)youdaoicibaDictYouDict[clapper 词源字典]
late 13c., 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.[clapper etymology, clapper origin, 英语词源]