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.
downspout
downstairs
downstream
downtime
downtown
downtrodden
downturn
downward
downy
dowry
dowse