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.
knowyourphrase.com - Common Phrases and Sayings - Meanings and Origins
Being upset over something that has already happened and cannot be changed.
Usually this phrase is said as "it's no use crying over spilt milk," which means that getting upset over certain things, like spilled milk, is not going to fix it.
As a frequent eater of cereal, it's safe to say that I have consumed a lot of milk during my life,
and yes, I've had to deal with the occasional spill. I can't say that I have ever cried over spilling
milk, though. However, even if I did, the crying would not help to fix a small problem like that. Instead, it would be better to simply accept that it happened and take steps in order to fix it, like grabbing some paper towels! And that's what this phrase means in a nutshell.
This expression is used in a book called Banking Under Difficulties from 1888, and its wording is similar to what we see today. To give you some context for the upcoming quote: a man had just been robbed of all his cash and by the looks of it, he was out and away from town. Depsite this, the man said:
"It was no use, however, crying over spilt milk."
The robbed man was in the quite the predicament, but he realized that in his current situation there really wasn't much he could do about what had happened, so he decided to press forward.
Apparently, though, this common saying goes back even further. There are plenty of sources citing that James Howell, a historian and writer, used the phrase in one of his literary works in 1659, called Paramoigraphy (Proverbs). I am unable to find the quote for myself, but supposedly it reads:
"No weeping for shed milk."
This would mean that this idiom is at least over 350 years old.
* My kids were upset because they had burnt their toast, but I told them that it's no use crying over spilt milk and to just make some more.
* Bill's car was destroyed in an accident earlier and he was angry about it, but later he came to the realization that there was no point in crying over spilled milk.