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A shifting crescent-shaped sand dune, a barchan; especially a coastal one in Peru or Chile.
Mid 19th century; earliest use found in The Southern Literary Messenger. From Spanish médano sandbank, sand dune, cognate with Portuguese medão sand dune, of uncertain origin, but probably ultimately from classical Latin mēta cone, heap, perhaps via a post-classical Latin neuter form corresponding to classical Latin mētula small heap from mēta + -ula.