"being behind, away from the front, in a backward direction," Middle English, from back (n.) and back (adv.); often difficult to distinguish from these when the word is used in combinations. Formerly with comparative backer (c. 1400), also backermore. To be on the back burner in the figurative sense is from 1960, from the image of a cook keeping a pot there to simmer while at work on another concoction at the front of the stove.
Old English stæger "stair, flight of steps, staircase," from Proto-Germanic *staigri (source also of Middle Dutch stegher, Dutch steiger "a stair, step, quay, pier, scaffold;" German Steig "path," Old English stig "narrow path"), from PIE *steigh- "go, rise, stride, step, walk" (source also of Greek steikhein "to go, march in order," stikhos "row, line, rank, verse;" Sanskrit stighnoti "mounts, rises, steps;" Old Church Slavonic stignati "to overtake," stigna "place;" Lithuanian staiga "suddenly;" Old Irish tiagaim "I walk;" Welsh taith "going, walk, way"). Originally also a collective plural; stairs developed by late 14c.
Others are reading
Definitions of backstairs
a second staircase at the rear of a building;
secret and sly or sordid; "his low backstairs cunning"- A.L.Guerard;