What does the name Millie mean?

Millie is a nickname of both Millicent and Camilla.

Millicent comes from the German Words "work, labour" and "strength."

Camilla is the feminine form of Camillus, which is ancient Roman in origin. The meaning is unknown.

Different answer:

My name's Amelia but everyone calls me Millie, accept my doctors and stuff. Millie is a nick name for loads of names but I know 4 Amelia's all called Millie. Hope this helps!