The word "butterfly" comes from the Old English word "buttorfleoge," which is a combination of the words for "butter" and "fly." The reasons for the name are obscure.
One theory regarding the origin of the word "butterfly" states that the word comes from the old idea that insects consumed butter or milk that is left uncovered. The insects were sometimes believed to be witches disguised as butterflies.
A simpler theory is that the pale yellow color of many butterflies reminded people of the color of butter. Another theory says the name comes from the color of butterflies' excrement, connecting it to the Dutch cognate "boterschijte."