noun
keeping time by tapping one's knees
Describes the action of slapping or tapping one's knees rhythmically, often to keep time with music or as a simple percussive accompaniment.
彼は歌に合わせて膝拍子をとった。
He kept time by tapping his knees along with the song.
盆踊りでは、膝拍子でリズムを刻む人もいる。
At the Bon dance, some people beat the rhythm by slapping their knees.
Compound of 膝 (hiza, 'knee') and 拍子 (byōshi, 'beat, rhythm'). The exact historical derivation is uncertain, but the word transparently describes the action of tapping one's knees to keep time.