A guess where you don't know the answer, but you do know something about the subject in general, so your answer will be better than just a WAG (Wild-Ass Guess).
John: Not sure. From my extensive knowledge of ancient Greek I can tell you "Drosophila" means "dew lover" and "melanogaster" means "fruit-eater". My EWAG would be some kind of fruit eating bug.
Bob (under breath): "What a Putz. I'll just look it up on wikipedia."