Who knows? The word is perhaps based on the old notion that witches disguised as butterflies consume butter or milk that is left uncovered.
Or simply because the pale yellow colour of many species’ wings suggests the colour of butter. Another theory connects it to the colour of their excrement, based on the Dutch cognate boterschijte.
Satisfied?
If not, please ponder the origin of the German word for butterfly: Schmetterling.
The verb schmettern means making a thunderous noise. So it cannot have a connection with butterflies.
But the Czech composer Smetana has.
Pfeifer’s etymological dictionary says the word comes from the southeastern German word Schmetten “cream,” borrowed from Czech smetana “cream.” It is based on the popular belief that witches fly in the form of butterflies to steal milk and cream, just like “butterfly”.

Just what I always wondered about! Thanks Eric… And papillon probably has nothing to do with papille!!!😃
Thank you, Gerry. Please note that Tim Nau answered your question about papillons.
If tin whistles are made of tin, what are fog horns made of?
And why aren’t French horns French?
Thanks, Michael, for a brilliant question.
Nigel Lewis in The Book of Babel says the ancient Greek word for butterfly was ‘psyche’, implying that it moves through the world like a soul freed from a body. The French ‘papillon’ is related to the Latin ‘palpebrae’ (eyelids), which flutter like butterflies, and the word can also refer, in modern slang, to a parking ticket fluttering on a windshield. Another language that connects the butterfly to fluttering is Spanish: ‘mariposa’ may mean ‘a bedside candle disturbed by a breeze’. Whatever the true etymology of ‘butterfly’ is, the gorgeous flutterer is one of the few insects humans like, and even seek to protect by way of folkloric taboos: As Blake says:
Kill not the moth nor butterfly,
For the Last Judgement draweth nigh.
Magnificent, Tim.
“Literature and butterflies are the two sweetest passions known to man.”
Vladimir Nabokov, ardent lepidopterist.
Very nice, David, and Vladimir.
I had learned long ago the original word was “flutterby” and was transposed into butterfly through common use. However, I’ve never found any authorities that back this up. But both Skeats and the OED hold that the original Anglo Saxon word was “buttor-fleoge”, literally “butter-fly” and so called from its excrement resembling butter; and more clearly, the Middle Dutch word was “boter-schijte”, literally a butter-voider! Not as entertaining or fanciful as some of the above explanations, but probably close to the truth? Anyway, as can be expected, this led me to a delightful hour of meandering through the OED as time fluttered by!
Thank you, Bobbi. An hour well spent. I can’t get over the discovery that butterflies are INSECTS! In my contribution, I avoided that word. Often, it is wise not to call things by their name.
Well, INSECTS is how WE HUMANS classify them. Who knows what they call themselves, or us for that matter? Nomenclature is a system, with certain limitations. And yes, I agree: an unexpected hour wading through the tall grasses of the OED was a pleasant and even relevant digression from my current immersion in the marshes of Halldor Laxness’ gorgeous “Independent People”, where the subjectivity of nomenclature abounds!
I always liked the “flutter-by” explanation, but alas my go-to reference for all things English-etymological (Eric Partridge’s Origins) doesn’t support it. He also doesn’t accept the folk-etymological “sparrow-grass” for asparagus. Boo-hoo. Sometimes a little learning is a saddening thing.