“Oh man, I lost that coupon I had for the coffee shop.”
“Well, no use crying over spilled milk.”
Crying over spilled milk (or “spilt” milk in UK English) means to be upset about something bad that happened that you can’t change. The bad thing that happened is usually a minor thing.
The image this idiom evokes for me is a young child who has spilled their glass of milk all over a table and starts to cry about it. Of course crying won’t undo the spill, and it won’t clean it up. It is a fruitless action. Also it is such a minor accident that full-blown crying seems overboard.
If you tell someone “not to cry over spilled milk” you are implying that the problem isn’t major. This can be used to comfort or chide someone who seems too worried about something minor.
“I wanted a chocolate shake, but the clerk said all they had left was vanilla.”
“Well, don’t cry over spilled milk. It’s just a drink.”
But be careful not to use it for things that are actually important.
“My girlfriend just dumped me.”
“No use crying over spilled milk. Plenty of other fish in the sea.” <==Could seem a little insensitive.