"mound, ridge, or hill of loose sand heaped up by the wind near the coast of a sea," 1790, from French, Middle Dutch or Middle Low German dune, all of which are perhaps from Gaulish *dunom (making it cousin to down (n.2) "small, rounded hill").
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. The English word is perhaps also partial a dialectal form of down (n.2). Dune buggy, "recreational motor vehicle designed for use on beaches," is attested by 1965.