Source : Webster's Revised Unabridged Dictionary (1913)

Upstairs \Up*stairs"\, adv.
   Up the stairs; in or toward an upper story.

Upstairs \Up"stairs`\, a.
   Being above stairs; as, an upstairs room.

Source : WordNet®

upstairs
     adj : on or of upper floors of a building; "the upstairs maid";
           "an upstairs room" [syn: {upstair}] [ant: {downstairs}]
     adv 1: on a floor above; "they lived upstairs" [syn: {up the stairs},
             {on a higher floor}] [ant: {downstairs}]
     2: with respect to the mind; "she's a bit weak upstairs" [syn:
        {in the head}, {in the mind}]