locking the barn door after the horse has bolted; too little, too late
A yojijukugo meaning to take action only after the damage is done; equivalent to the English proverb. The literal meaning is 'mending the pen after the sheep are lost'.
Strengthening safety measures after an accident has occurred is like locking the barn door after the horse has bolted.
Borrowed from Chinese 亡羊補牢, literally “mend the enclosure after losing the sheep,” an expression recorded in the 戦国策.