down (adv.) Look up down at Dictionary.com
late Old English shortened form of Old English ofdune "downwards," from dune "from the hill," dative of dun "hill" (see down (n.2)). A sense development peculiar to English.

Used as a preposition since c.1500. Sense of "depressed mentally" is attested from c.1600. Slang sense of "aware, wide awake" is attested from 1812. Computer crash sense is from 1965. As a preposition from late 14c.; as an adjective from 1560s. Down-and-out is from 1889, American English, from situation of a beaten prizefighter. Down home (adj.) is 1931, American English; down the hatch as a toast is from 1931; down to the wire is 1901, from horse-racing. Down time is from 1952. Down under "Australia and New Zealand" attested from 1886; Down East "Maine" is from 1825.
down (n.1) Look up down at Dictionary.com
"soft feathers," late 14c., from Old Norse dunn, perhaps ultimately from PIE root *dheu- (1) "to fly about (like dust), to rise in a cloud."
down (n.2) Look up down at Dictionary.com
Old English dun "down, moor; height, hill, mountain," from Proto-Germanic *dunaz- (cognates: Middle Dutch dunen "sandy hill," Dutch duin), "probably a pre-insular loan-word from Celtic" [Cambridge Dictionary of English Place-Names], in other words, borrowed at a very early period, before the Anglo-Saxon migration, from PIE root *dheue- "to close, finish, come full circle." Meaning "elevated rolling grassland" is from c.1300.

The non-English Germanic words tend to mean "dune, sand bank" (see dune), while the Celtic cognates tend to mean "hill, citadel" (compare Old Irish dun "hill, hill fort;" Welsh din "fortress, hill fort;" and second element in place names London, Verdun, etc.). German Düne, French dune, Italian, Spanish duna are said to be loan-words from Dutch.
down (v.) Look up down at Dictionary.com
1560s, from down (adv.). Related: Downed; downing.