spilled milk就是“撒出来的牛奶”，这句话的字面意思就是“牛奶洒出来了，哭也没用”，引申一下就是“事已至此，哭也没用”。

【例句】
There is no regret medicine in the world. Don't cry over spilled milk.
世界上没有后悔药，不要为打翻的牛奶哭泣。

I know that you have lost your purse, but since it had happened, let it go, please don't cry over spilled milk.
我知道你丢了钱夹，不过既然这件事已经发生了，就让它去，不要为打翻的牛奶流泪。