c. 1600, "peculiar person, person of a type seldom encountered," 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. A phrase used of Horace's peacock (a Roman delicacy), Juvenal's black swan ("Rara avis in terris, nigroque simillima cygno"). A figure perhaps natural to the superstitious Romans, who divined by bird-watching.