Well, the fly bit of this word is easy to understand. But why butter?
Is it because, as some say, the first butterfly of the English year is bright yellow (unless, of course, you happen to see one of the mostly brown ones that occur at the same time)?
Is it to do with flutter by?
The Oxford English Dictionary comes up with the Dutch word boterschijte and wonders if it's because the insect's poo looks like butter.
The trouble is that my lepidopterist husband has just been showing me examples of moth and butterfly poo (marriage, eh?) and the colour varies greatly. In any case, I'm pretty certain a butterfly's poo isn't the main thing you'd notice about it.
The Collins dictionary says that butterflies are said to steal milk and butter. Whoever first said this is, however, BONKERS because whilst butterflies might sip at honey or rotten fruit (and they love cheap lager) I've never seen one go anywhere near a milk jug or a butter dish.
Wilhelm Oehl's idea was that, when people were first inventing words, the word for butterfly was buto or boto, which made a picture in sound of their flapping wings.
Later, people tried to make sense of this sound with stories. The Russians came up with witches turning themselves into butterflies because their word babochka means witch. We came up with various butter stories.
And what's true?
What's true is that the sight of a butterfly lifts the heart.
And I don't know why that is, either.