noun
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'.
事故が起きてから安全対策を強化するのは亡羊補牢だ。
Strengthening safety measures after an accident has occurred is like locking the barn door after the horse has bolted.
From Chinese 亡羊補牢 (wáng yáng bǔ láo), literally 'mend the pen after the sheep are lost'. The exact historical derivation is uncertain, but it is a well-known yojijukugo in Japanese.