Old English clæppan "to throb, beat," common West Germanic, echoic (cf. Old Frisian klapa "to beat," Old Norse klappa, Old High German klaphon, German klappen, Old Saxon klapunga). Meaning "to strike or knock" is from c.1300. Meaning "to make a sharp noise" is late 14c. Of hands, to beat them together to get attention or express joy, from late 14c. To clap (someone) on the back is from 1520s. Related: Clapped; clapping.