"in a descending direction, from a higher to a lower place, degree, or condition," late Old English shortened form of Old English ofdune "downwards," originally of dune "off from (the) hill," from dune "from the hill," dative of dun "hill" (see down (n.2)). The "hill" word is general in Germanic, but this sense development is peculiar to English. As a preposition, "in a descending direction upon or along," from late 14c.
To be down on "express disapproval of" is by 1851. Down home is from 1828 as "in one's home region," as an adjective phrase meaning "unpretentious" by 1931, American English. Down the hatch as a toast is from 1931. Down to the wire is 1901, from horse-racing.
Down Under "Australia and New Zealand" attested from 1886; Down East "Maine" is from 1825; Down South "in the Southern states of the U.S." is attested by 1834. Down the road "in the future" is by 1964, U.S. colloquial. Down-to-earth "everyday, ordinary, realistic" is by 1932.
"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.
"long, narrow ridge or hill of sand, gravel, and boulders," in areas of Ice Age glaciation and formed somehow by the movement of the ice, 1833, a diminutive of earlier drum (1725) "ridge or long, narrow hill," often separating two parallel valleys, from Gaelic and Irish druim "back, ridge." Somewhat similar to, though different in origin (probably) from, an esker, but their exact nature is not quite understood.
"mound, hill, grave-mound," Old English beorg (West Saxon), berg (Anglian) "barrow, mountain, hill, mound," from Proto-Germanic *bergaz (source also of Middle Dutch berch, Old Saxon, Old High German berg "mountain," Old Frisian berch, birg "mountain, mountainous area," Old Norse bjarg "rock, mountain"), from PIE root *bhergh- (2) "high," with derivatives referring to hills and hill-forts.
Obsolete by c. 1400 except in place-names and southwest England dialect; revived by modern archaeology. Meaning "mound erected over a grave" was in late Old English. Barrow-wight first recorded 1869 in Eirikr Magnusson and William Morris's translation of the Icelandic saga of Grettir the Strong.
In place-names used of small continuously curving hills, smaller than a dun, with the summit typically occupied by a single farmstead or by a village church with the village beside the hill, and also of burial mounds. [Cambridge Dictionary of English Place-Names]