Downstairs means on or to a lower floor. Go downstairs, the downstairs bathroom, they live downstairs.
It works as an adverb — come downstairs — and as an adjective before a noun: the downstairs window. As a noun it means the lower floor generally: the whole downstairs was flooded. Note that no preposition is needed: go downstairs, not go to downstairs. In British social history, downstairs meant the servants' part of a house, against upstairs for the family, which is where the phrase upstairs, downstairs comes from.
1adverbdownstairsOn or to a lower floor.
“She went downstairs to see what the noise was.”
2adjectivelowerOn the lower floor.
“The downstairs window had been left open all night.”
Too few real examples of this word to estimate which sense is commonest.
downstairs is a position in a building.
Middle English
doun
English
down
English, today
downstairs
Built from
From down- + stair + -s. down comes from Middle English doun. From Middle English doun, doune (“down”), from Old English dūne (“down”), aphetic form of adūne (“down, downward”), from earlier ofdūne (“down”) literally “off the hill”, from of (“of, off of”) + dūn (“hill, mount, dune, down”). More at Etymology 2 below. For the development from directional phrases to prepositions, compare dene, dāle, whence dal. Compare also stq, del. Cognate with Scots doon (“down”).
Etymology from Wiktionary, CC BY-SA