Upstairs means on or to a higher floor. Go upstairs, the bathroom is upstairs, an upstairs window.
It takes no preposition: go upstairs, never go to upstairs, and that is the point learners most often get wrong. It works as an adverb, as an adjective before a noun, and as a noun with the: the upstairs of the house. Downstairs is its exact opposite and behaves identically. Note the informal figurative use meaning senior management — a decision from upstairs — and Upstairs, Downstairs as a phrase for the two worlds of a large old household.
1adverbupstairsOn or to a higher floor.
“She went upstairs and did not come down for an hour.”
2adjectivehigherOn a higher floor.
“The upstairs windows have not been cleaned in years.”
3nounseniorSenior management, informally.
“It has to be signed off upstairs.”
Too few real examples of this word to estimate which sense is commonest.
upstairs is a position in a building.
Middle English
steire
English
stair
English, today
upstairs
From up- + stair + -s. stair comes from Middle English steire. From Middle English steire, staire, stayre, stayer, steir, steyre, steyer, from Old English stǣġer (“stair, staircase”), from Proto-West Germanic staigri, from Proto-Germanic staigriz (“stairs, scaffolding”), from Proto-Indo-European steygʰ- (“to walk, proceed, march, climb”). Cognate with Dutch steiger (“a stair, step, wharf, pier, scaffolding”), Middle Low German steiger, steir (“scaffolding”), German Low German Steiger (“a scaffold; trestle”). Related to Old English āstǣġan (“to ascend, go up, embark”), Old English stīġan (“to go, move, reach; ascend, mount, go up, spring up, rise; scale”), German Stiege (“a flight of stairs”). More at sty.
Etymology from Wiktionary, CC BY-SA