c.1600, "peculiar person," from Latin rara avis, literally "strange bird," from rara, fem. of rarus "rare" (see rare, adj.1) + avis "bird" (see aviary). Latin plural is raræ aves. Horace's peacock (a Roman delicacy), Juvenal's black swan ("Rara avis in terris, nigroque simillima cygno").