Fly2  n. (pl. Flies) 1 insect with two usu. Transparent wings. 2 other winged insect, e.g. A firefly. 3 disease of plants or animals caused by flies. 4 (esp. Artificial) fly as bait in fishing.  like flies in large numbers (usu. Of people dying etc.). No flies on (him etc.) Colloq. (he is) very astute. [old english]