Uh, yuck?
The name is derived from Middle English buterflie, butturflye, boterflye, from Old English butorflēoge, buttorflēoge, buterflēoge, perhaps a compound of butor (beater), mutation of bēatan (to beat), and flēoge (fly).[2]
Alternate etymology connects the first element to butere (butter) as the name may have originally been applied solely to butterflies of a yellowish or butter-colour. This may have merged later with the belief that butterflies ate milk and butter (compare Middle High German molkendiep -literally “milk-thief”; Modern German Molkendieb and Low German Botterlicker - literally “butter-licker”), or that they excreted a butter-like substance (compare Middle Dutch boterschijte - literally “butter-shitter”, also Middle Dutch botervliege, Dutch botervlieg, German butterfliege).