Butterfly:

So why is a butterfly called a butterfly? I was wondering, so I looked it up. The OED reports that the origin is unknown, but notes that Hensleigh Wedgwood’s mid-1800s A Dictionary of English Etymology linked it to the old Dutch boterschijte, which apparently referred to the color of the insect’s excrement. The Online Etymology Dictionary notes this theory as well, though also reports two other possibilities — “the old notion that the insects (or witches disguised as butterflies) consume butter or milk that is left uncovered” and “less creatively, simply [that] the pale yellow color of many species’ wings suggests the color of butter.”

All I can say is that if boterschijte is the source, I’m glad the word has evolved since then.