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backstairs [backstairs 词源字典]: "stairs at the back of a structure," 1650s, from back (adj.) + stairs (see stair). Figurative use is attested earlier (1640s).[backstairs etymology, backstairs origin, 英语词源]
downstairs (adv., adj.): 1590s, from down (adv.) + stairs (see stair).
upstairs (adv.): 1590s, from up (adv.) + stairs (see stair). As an adjective from 1782. The noun is first attested 1872. Adjectival meaning "characteristic of upstairs life" (in private rooms of a household, as opposed to servants' quarters) is recorded from 1942.
He [Halifax] had said he had known many kicked down stairs, but he never knew any kicked up stairs before. [Gilbert Burnet, supplement to "History of My own Time," from his original memoirs, c. 1697]