The origin of the word butterfly is not clear. Since the term is pre-8th century we have little to go on. Some people like to think it came from a turn on the words 'flutter by', but that is most likely not the origin.
Our version comes from the Anglo-Saxon word 'butterfloege' (butere and fleoge) and some guess that a common yellow butterfly was the reason for this name as it reminded them of butter flying. In Russian they are called 'little soul' and the Ancient Greeks call them 'Psyche' which also means soul.
Some think the name came from butterfly droppings which are a yellow drop. One folktale is that witches took the shape of butterflies to steal milk and butter when they came into the house.
Whatever, we do know that their name does not really reflect their beauty or fancy or mysterious metamorphosis each summer.
(Still waiting for that rare Monarch.)