Q From David Powell: Could the word butterfly, which has no obvious connection with butter (outside of its name), possibly be an ancient corruption of flutter by, which is exactly what the creature does? I can imagine a child mispronouncing it thusly and the result sticking in our language.
A This has been suggested as a possible source of the name — the inversion is terribly seductive. There’s a big problem, though. J B S Haldane once spoke sadly of “a beautiful theory, slain by an ugly fact” and I’m about to tread all over the idea with my size nine clodhoppers of evidence. The word can be traced right back to the Old English buterflege, found in a glossary of about the year 700. Nowhere along its trip from those times to today does it ever appear in the inverted form. The word does indeed seem to be butter plus fly. Words in some other European languages also contain the same idea, such as the German Schmetterling, whose ultimate root is a Czech word for cream.
Why butter? We wish we knew for sure. Some authorities suggest the link comes from the insect’s yellow faeces. Others point to the old belief that butterflies like to land on milk or butter left uncovered, or even that fairies and witches took on the form of butterflies at night to steal butter from the dairy. Nice stories, but lacking rather a lot on the firm-evidence front.
Stephen Potter pointed out in his book Pedigree: Words From Nature that the most common British butterflies around habitations, such as the small white and cabbage white, are cream or pale yellow in colour, the usual colour of butter in the days before manufacturers started to add dyes to it. This would be my first choice for an explanation, too, so making a butterfly a butter-coloured flying thing.