"vertical column between the lights of a window," 1560s, metathesis of Middle English moyniel (early 14c.), from Anglo-French moinel, noun use of moienel (adj.) "middle," from Old French meien "intermediate, mean" (see mean (adj.)). Related: Mullioned.
An unattractive person: Either you're a mullion, a dog
[1970s+ Baseball; origin unknown]