Don’t shut the stable door after the horse has bolted

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To have tried to prevent something happening but done so too late to prevent damage being done.

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John Gower’s poem Confessio Amantis published in 1390 and it may be that the proverb was in use in everyday language for some time before that: For whan the grete Stiede Is stole, thanne he taketh hiede, And makth the stable dore fast.