油断
ゆだん · yudan · JLPT N2 · noun
negligence
🛢️ 油 oil (slippery) + 断 cut off: your guard slides away, leaving you unready
Sounds like 'you, done': careless for a moment and you're done for. Oil is slippery, so a lapse slides in unnoticed.
油 — water radical 氵 + 由 (yóu, phonetic)
油 is a phono-semantic compound. The 氵 radical signals a liquid, while 由 supplies the sound. Originally referring to a river name, it was extended to mean oil or fat — any thick, flowing liquid — which became its dominant modern sense.
断 — thread 㡭 (silk bundle) + axe 斤
Originally written with 㡭 (entangled silk threads) on the left and 斤 (axe) on the right, depicting an axe cutting through bundled threads. From this concrete image of severing came the meanings 'to cut off,' 'to break,' and by extension 'to decide' or 'judge'—as in resolving a matter decisively.