Being too late to take action; having missed the final chance so that recovery is impossible or difficult.
もっと早く治療を受けていれば助かったかもしれないが、今は手遅れだ。
If he had received treatment earlier he might have been saved, but now it's too late.
準備不足で試験に落ちたら、それは手遅れだ。
If you fail the exam because you didn't prepare, then it's too late.
問題が小さいうちに対処しなければ、手遅れになることが多い。
If you don't deal with problems while they're small, they often become too late to fix.