马后炮本是一个象棋术语，意思是在马后面加一颗炮，将他们的军（check, put the other players king under direct attack）。后来大家用马后炮比喻事后采取手段，无济于事；或事情发生后才提出批评或建议，表示自己有先见之明的人（a person who criticizes or suggests alternative courses of action from a position of hindsight after the event in question），也称事后诸葛亮（second guesser, be wise after the event）。
例句：
考试都完了，你才说帮我复习功课，这岂不是马后炮吗？
You come and offer to help with my lessons only after the examination. Isnt that a bit too late?
他是个马后炮。
He is a second-guesser.
事后诸葛亮每人都会做。
It is easy to be wise after the event.