Millie is a shortened form of the name Millicent.
While Millie is a popular name on its own, it's often used as a nickname for girls named Millicent.
Here are some other names Millie could be short for:
- Camilla
- Amelia
- Mildred
- Melinda
- Millison
It's important to note that Millie can also be a standalone name, not necessarily a nickname.