deerfly Syllabification: (deer·fly)Pronunciation: /ˈdi(ə)rˌflī/Definition of deerfly noun 1a bloodsucking horsefly that attacks humans and other large mammals. It can transmit various diseases, including tularemia. Genus Chrysops, family Tabanidae: several species, including C. callidus, widespread throughout North America 2a bloodsucking louse fly that is a parasite of deer. It loses its wings on finding a host, and the female gives birth to fully grown larvae. Lipoptena cervi, family Hippoboscidae