stair

O.E. stæger "flight of steps," also "a single step," from P.Gmc. *staigri (cf. O.N., O.Fris. stiga, M.Du. stighen, O.H.G. stigan, Ger. steigen, Goth. steigan "to go up, ascend;" O.E. stigan "to climb, go;" Ger. Steig "path," O.E. stig "narrow path"), from PIE *steigh- "go, rise, stride, step, walk" (cf. Gk. steikhein "to go, march in order," stikhos "row, line, rank, verse;" Skt. stighnoti "mounts, rises, steps;" O.C.S. stignati "to overtake," stigna "place;" Lith. staiga "suddenly;" O.Ir. tiagaim "I walk;" Welsh taith "going, walk, way"). Originally also a collective plural; stairs developed by 1398. OED says stair still is ordinary in Scotland where flight of stairs would be used elsewhere. Staircase is from 1624, originally the enclosure of the stairs; stairway is from 1767.