This was also a nickname for Emily or Amelia, especially during the 1900's-1920's I think. It sounds dated for Amelia but to me, it sounds great for Millicent, Camilla and other names starting with 'Mil.' 'Mil' also means strength, determination, admiring so Millie can mean 'admirable strength.'
― Anonymous User 3/13/2006
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Millie can also be a shortname for Milene. Pronounced M-I-L-EE-N not MY-LYNN.