膝ひざを打うつ

expression, godan verb (-tsu)

slap one's knee (in sudden realization or admiration)

Idiomatic expression for the gesture of slapping one's knee when struck by a good idea or impressed by something clever. The literal meaning is the physical action, but it is almost always used figuratively.

• 彼かれの名めい案あんに思おもわず膝ひざを打うった。

  I spontaneously slapped my knee at his brilliant idea.

• その解かい決けつ策さくを聞きいて、一いち同どうは膝ひざを打うって感かん心しんした。

  Hearing the solution, everyone slapped their knees in admiration.