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.
彼の名案に思わず膝を打った。
I spontaneously slapped my knee at his brilliant idea.
その解決策を聞いて、一
Hearing the solution, everyone slapped their knees in admiration.