Milly is a related term of millie.

Milly is a alternative form of millie.

As proper nouns the difference between millie and milly

is that millie is a diminutive of the female given names Mildred, Millicent, Emily, Amelia and Camilla, also used as a formal given name while Milly is a diminutive of the female given names Mildred, Millicent, Emily and Camilla, also used as a formal given name.

As a noun millie

is (Northern Ireland, 19th Century, pejorative or affectionate) a mill-worker, usually a young, working class woman working in the factories of Ireland's linen industry.