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negligence; carelessness; inattention; unpreparedness
油断していると大きな失敗をするよ。
If you let your guard down, you'll make a big mistake.
試験前に油断して勉強しなかった結果。
The result of being careless and not studying before the exam.
油断大敵という言葉を忘れないでね。
Don't forget the saying that carelessness is the enemy.