Four-Character Idiom · Reflection & Mortality · Kyojuro Rengoku
Meaning
A moment's slack attention is the greatest enemy of any practice — used as a self-reproach more often than as a warning to others.
少しの気の緩みが大きな失敗を招くことへの強い自戒。
The Kyojuro Rengoku line
Verbatim from the source article
「うたた寝している間にこんな事態になっていようとは！！」
"SO THIS IS WHAT HAPPENED WHILE I WAS DOZING?"
Why it fits Kyojuro Rengoku
Caught off-guard by the dream-trap on the Mugen Train, Rengoku's first reaction is yudan-taiteki: he indicts his own lapsed attention rather than the demon's craft. The reflex is a Hashira-grade self-evaluation reflex.
Cultural note
Origin in classical / Buddhist / Confucian tradition
Originally a Buddhist term — 油断 came from the parable of the disciple who let the oil lamp in the temple run out by inattention, an unforgivable lapse of vigil.
Sources
- デジタル大辞泉 — 油断大敵 (dictionary)
- 精選版 日本国語大辞典 — 油断大敵 (dictionary)