It refers to the process of being ground down, worn out, and changed by a challenging or traumatic experience. Imagine grain being processed in a mill—it comes out changed and often smaller than it was before.
Here are some examples of how it's used:
* "After losing his job and going through a divorce, he's been through the mill."
* "She's been through the mill with her health problems, but she's still strong."
* "This project has been through the mill several times before we finally got it approved."
Similar idioms:
* Seen it all
* Been there, done that
* Gone through the wringer
* Paid his dues
Essentially, "been through the mill" implies someone has faced significant challenges and come out the other side, potentially changed by the experience.