close the stable door after the horse has bolted

• close the barn door after the horse has bolted, lock the stable door after the horse has bolted, shut the stable door after the horse has bolted, close the stable door after the horse has been stolen, lock the stable door after the horse has been stolen

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close the stable door after the horse has bolted (third-person singular simple present closes the stable door after the horse has bolted, present participle closing the stable door after the horse has bolted, simple past and past participle closed the stable door after the horse had bolted)

1. (idiomatic) To attempt to prevent a problem that has already occurred; to design and implement a preventive mechanism too late (after the first failure instead of before).
   • 2019 May 15, Ian C. Lucas, “WhatsApp Data Breach”, in parliamentary debates (House of Commons)‎, column 242:

     We are now in an election period, with WhatsApp and closed Facebook groups being used, as we speak, in electoral campaigning, but the law has not changed since the DCMS Committee, on which I serve, raised these issues. We have yet another instance of shutting the door after the horse has bolted.

• barn door, horsebarn

• late in the day
• that ship has sailed
• too little, too late
• Scarborough warning
• sell past the close

• “close the stable door after the horse has bolted”, in Collins English Dictionary.
• Gary Martin (1997–) “Close the stable door after the horse has bolted”, in The Phrase Finder.