It's a lovely story that claims the word �butterfly' comes from the fact that one may be seen to �flutter by'. ..ie that it is just a metathesis or letter-swop along Spoonerish lines.
Sadly that's all it is...a story. Even in Old English days, the words �butter' and �fly' were joined to create �butorfleoge' as the creature's name. There may be a connection with a Dutch dialect word for butterfly...�boterschijte', which suggests a relationship between butter and butterfly excrement, both being yellow. Unfortunately - though probably more accurate - that's not quite such a pretty picture as flutter-by!
Another theory is that �butor' was also the Old English name for the finest of pasture-land. Obviously, many butterflies would be attracted to such grass and the cattle on it would produce the finest of butter...hence the name of the dairy product.
A strange thing is the fact that the word �butterfly' itself seems to come from different roots in each of the other four major European languages...French = papillon, Italian = farfalla, Spanish = mariposa and German = Schmetterling.