油断
ゆだん negligence; carelessness
Why it means this
油 (oil) + 断 (cutting off) — perhaps from a Buddhist tale of a king who would lose his life if he let a bowl of oil spill; carelessness, letting one's guard down (油断大敵; 油断も隙もない).
Breakdown
Each character links to its full entry — origin story, stroke order, and readings.