The noun fly is one of those unusual words that originated with the verb to fly, meaning of course, to soar, flap wings, break the bonds of gravity, etc. For reasons largely unknown, this word then, describing the insect, comes from the Germanic Fliege, meaning, to fly. This, then, came to mean in the Old English, any winged insect, including butterflies. The word has had so many permutations through Norse and Old as well as Middle English that the true origin as the insect has been lost. (With help from Online Etymology Dictionary).