Why is the ninth month called September?
1790, from French, Middle Dutch or Middle Low German dune, all perhaps from Gaulish *dunom (thus related to down (n.2)). The French dune "sand hill" (13c.) is held by Diez to be an Old French borrowing from Dutch duin or some other Germanic source. Italian and Spanish duna are from French. Dune buggy attested by 1965.
A hill or ridge of wind-blown sand. Dunes are capable of moving by the motion of their individual grains but usually keep the same shape. See more at barchan dune, draa, longitudinal dune, seif dune, transverse dune.