Means getting upset about something that’s already done and can’t be fixed, like yeah, the damage is done. You can have a little moan, but sitting there sobbing won’t undo it, it just makes you late for the next thing as well. Classic for missed chances, silly mistakes, or any drama that needs dropping.
"I left my keys on the bus like an absolute muppet. Ah well, no point crying over spilt milk, I’ll grab a brew and ring the locksmith before Mum starts tutting"