(Latin, a rare bird). A phenomenon; a prodigy; a something quite out of the common course. Black swans are now familiar to us; they are natives of Australia, and have given its name to the “Swan river.” At one time a black swan was emphatically a rara avis.
“Rara avis in terris nigroque simillima cygne.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894