1(Chinese chess) moving the cannon after moving the horse away
2(fig.) a useless action or comment; sth done after it's too late
Example
shì qing事情yǐ jīng已经dìng定xià lai下来liǎo了，wǒ men我们zài再zěn me怎么nǔ lì努力yě也wú jì yú shì无济于事liǎo了，yī qiè一切dōu都shì是mǎ hòu pào马后炮liǎo了。
事情已经定下来了，我们再怎么努力也无济于事了，一切都是马后炮了。
The matter can't be changed. No matter how hard we try, everything is fruitless.