c'est de la moutarde après dîner

IPA(Pronunciation)

figuratively

（比喩的に）馬が逃げ出した後に馬小屋の扉を閉めるようなものだ

English Meaning

(figuratively) it's like closing the stable door after the horse has bolted

後手に回ってミスを正そうと急いだが、結局、逃げた馬の後に厩舎の扉を閉めるようなものだった。

Trying to fix his mistake after the fact, he rushed into action, but in the end, it's like closing the stable door after the horse has bolted.