Downstairs
来自Big Physics
wiktionary
down- + stair + -s
etymonline
downstairs (adv.)
"down the stairs, below, to or on a lower floor," 1590s, from the prepositional phrase; see down (adv.) + stair. As an adjective, "pertaining to, relating to, or situated on the lower floor of a house," by 1819. As a noun, "the downstairs part of a building," by 1843.
