Millie

Depends on the longer name; through Mildred, “gentle strength”.

Millie is an English feminine name with roots as an affectionate short form, notably of Mildred and Millicent. It therefore has no single meaning that fits every family’s choice. Through Mildred it connects to the Old English elements “gentle” and “strength”. Pronounced /ˈmɪl.i/ in English, Millie moves from a soft m through a flowing l to a bright ee ending. It can feel warm, informal and gently vintage. As a given name in its own right, it keeps that familiar sound without requiring a longer form. When chosen to honour a Mildred or Millicent, the particular family connection adds its own layer of meaning.

Etymology: An English affectionate short form of names including Mildred and Millicent, which have distinct older histories.