No one knows for sure, since the word has been in the English language for centuries (the word was "buterfleoge" in Old English, which means butterfly in our English today). Because it is such an old word, we don't really know who or when someone said "That 'thing' over there is a 'butterfly'." One story is that they were named so because it was thought that butterflies, or witches that took on the shape of butterflies, stole milk and butter. (Someone else wondered if the word was really meant to be "flutter-by" ). In other languages, the word for our fluttery friends has no such derivation as "butter" + "fly". One can only speculate on why the English language uses such an unlikely name.