1560s, "stepped on, trampled upon," from down (adv.) + past participle of tread (v.). Figurative sense of "oppressed, tyrannized" is from 1590s. To tread down in the sense of "overcome, destroy" is from late 12c.
"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.
Old English tredan "to tread, step on, trample; traverse, pass over" (class V strong verb; past tense træd, past participle treden), from Proto-Germanic *tred- (source also of Old Saxon tredan, Old Frisian treda, Middle Dutch treden, Old High German tretan, German treten, Gothic trudan, Old Norse troða), from PIE *der- (1) "assumed base of roots meaning 'to run, walk, step'" [Watkins]. Related: Trod; treading. To tread water in swimming, "to move the feet and hands regularly up and down while keeping the body in an erect position in order to keep the head above the water" is attested by 1764.