 The phrase "black swan" derives from a Latin expression; its oldest known occurrence is from the 2nd-century Roman poet Juvenal's characterization in his "Satire VI" of something being ""rara avis in terris nigroque simillima cygno"" ("a rare bird in the lands and very much like a black swan").When the phrase was coined, the black swan was presumed not to exist.The importance of the metaphor lies in its analogy to the fragility of any system of thought.