A four-character proverb warning that carelessness is a great enemy; do not let your guard down.
試験直前だからこそ油断大敵だ。最後まで集中しよう。
All the more reason not to let your guard down right before the exam. Stay focused until the end.
山道では一瞬の油断大敵で事故につながることがある。
On mountain paths, a momentary lapse can lead to an accident — carelessness is a great enemy.
新型ウイルスの流行が続く中、油断大敵という言葉を忘れてはいけない。
With the new virus still spreading, we must not forget the saying 'carelessness is the greatest enemy.'