To take precautions or remedial action only after damage has been done or an accident has occurred; to act too late.
经过这次教训，他们终于意识到安全防范的重要性，但大家都觉得有点贼去关门，为时已晚。
After this lesson, they finally realized the importance of security precautions, but everyone felt it was a case of locking the stable door after the horse had bolted, and it was too late.
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