How did butterfly get its name? There are multiple theories. In German language old or local names for butterflies are “Molkendieb” (“whey thief”) and “Botterlicker” (“butter licker”). It was believed that many insects, including butterflies, stole milk and butter. Other ancient hypothesis claimed that butterflies were witches temporarily shapeshifted into insects, and it’s the witches who stole the butter.

But the most probable origin of the word butterfly is a biological one. Most butterflies have yellow colored excrements, similar to butter. Dutch language word for butterfly is “boterschijte”, which literally means “butter-pooper” or “butter-shitter”. “Schijten” is a vulgar Dutch word meaning “shitting”.

Fact sources:
Wiktionary
Grammarphobia