Being upset over something that has already happened and that cannot be changed.
Usually said as "it's no use crying over spilt milk," which implies that being sad over certain things, like spilled milk, is not going to fix it.
As a frequent eater of cereal, I've consumed a lot of milk in my day and have had to deal with the occasional spill, though I can't say I've ever cried over it. Unless the bowl had fallen on my foot, there wouldn't really be much of a reason to. My tears are not going to help me clean up the mess! That's sort of the backbone of what the idiom means: don't waste time moping over something that's happened and that you have no control over, but instead, continue moving forward.
That's what a man had to do in a book called Banking Under Difficulties from 1888. To give you some context, he was in quite the predicament, because he had just been robbed of all his cash! Even so, he says:
"It was no use, however, crying over spilt milk."
Indeed, the thief was long gone and no where to be seen.
This popular saying, however, goes back even further. There are plenty of sources on the internet citing that James Howell, a historian and writer, used the phrase in one of his literary works in 1659, called Paramoigraphy (Proverbs). Supposedly it reads:
"No weeping for shed milk."
The origins for this phrase, thus, look to be at least over 350 years old.
Note: Tracking the origins of phrases is, most of the time, impossible! What's provided are theories that may be plausible to how a phrase originated, but not necessarily so.
In addition, quotes that contain a particular phrase may be taken from old newspapers, poems, or books that were written centuries ago, but this by no means confirms that the phrase originates from said newspapers, poems, or books. In all likelihood, if an expression is being used in a newspaper, it's probably already a well known saying and is thus, from an older time.
* The worst is behind us now, so there's no need to cry over spilt milk.
* Bill was angry that he had destroyed his car in an accident earlier, but he later realized there was no point in crying over spilled milk and calmed down.