the.com/upstairs

means The upper floor(s) of a building, reached by stairs — and, by extension, the people or business of higher authority or rank.

from A straightforward compound of 'up' and 'stairs,' formed in Middle English once staircases became common enough in houses to need a word for the direction they went. The literal sense — the floor above — came first; the figurative sense, for bosses, management, or 'those in charge' (as opposed to the 'downstairs' of servants or staff), grew naturally out of the old architecture of great houses, where masters lived up top and servants worked below.