stair n.

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.