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.
Not much is known about the origin of this phrase. The earliest that I could find the expression
in writing is in a book called Banking Under Difficulties from 1888, and its wording is similar to what we see today. To give some context on 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. Quite the predicament to be in. However, depsite the circumstances, the man said:
"It was no use, however, crying over spilt milk."
The robbed man was in a difficult situation, but he realized that there really wasn't much he could do about it at that time other than moving forward.
Apparently, though, this common saying goes back even further. It's believed that James Howell, a historian and writer, used the phrase in one of his literary works in 1659, called Paramoigraphy (Proverbs). I don't have access to the book to see 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 heavily damaged in an accident earlier and was no longer functional. He was angry about what had happened, but he soon realized that there was no point in crying over spilled milk.