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Phrase(s): Shut the stable door after the horse has bolted. [and] Lock the stable door after the horse is stolen.
Prov. To try to prevent something that has already happened; to act too late. • When Ray heard that the bank had failed, he tried to withdraw his money, but there was no money to withdraw. He was shutting the stable door after the horse had bolted. • Jenny has stopped smoking since the doctor told her that her lungs were in bad shape, but I’m afraid she’s locking the stable door after the horse is stolen.
McGrawhill's American Idioms And Phrasal Verbs