膝を打つ

• Reading: ひざをうつ
• Romaji: hiza o utsu
• Kanji breakdown: 膝 (shitsu/hiza) — knee; 打 (da/u) — hit, strike
• Pronunciation: /hi.za o u.t͡sɯ/

Meaning

To slap one's knee in sudden realization; to be struck with admiration; to exclaim 'that's it!' at a brilliant idea.

An idiomatic expression using the Group 1 (godan) verb 打つ (utsu, to hit). Literally means 'to strike one's knee' — a spontaneous physical gesture accompanying a flash of insight or admiration. Common in literary and formal contexts when describing the moment of sudden realisation or when someone's idea is so apt it generates immediate enthusiasm. Less common in casual speech but universally understood.

Examples

1. その提案を聞いて、思わず膝を打った。 Upon hearing that proposal, I involuntarily slapped my knee in agreement.
2. 彼の発想のユニークさに膝を打ち、すぐに採用を決めた。 Struck by the uniqueness of his idea, I slapped my knee and immediately decided to adopt it.
3. 難問の答えが突然ひらめき、思わず膝を打つような瞬間だった。 The answer to the difficult problem suddenly came to me — it was one of those knee-slapping moments of realization.

Usage Guide

Context: ideas, admiration, realisation, business

Tone: enthusiastic

Origin & History

From the traditional gesture of slapping one's knee — a spontaneous bodily response to excitement or surprise. In traditional Japanese seating on the floor, striking the knee is a natural way to express sudden strong emotion or enthusiastic approval.

Cultural Context

Era: Traditional–Modern

Generation: Adults

Social background: Universal