No hay ninguna relación entre las palabras:
De Mari, apóc. de María, y posa, 2.ª pers. de sing. del imper. de posar.
American Heritage Dictionary of the English Language, Fourth Edition:
Is a butterfly named for the color of its excrement or because it was thought to steal butter? It is hard to imagine that anyone ever noticed the color of butterfly excrement or believed the insect capable of such theft. The first suggestion rests on the fact that an early Dutch name for the butterfly was boterschijte. The second is based on an old belief that the butterfly was really a larcenous witch in disguise.