Source : Webster's Revised Unabridged Dictionary (1913)

Backstairs \Back"stairs`\, Backstair \Back"stair`\, a.
   Private; indirect; secret; intriguing; -- as if finding
   access by the back stairs.

         A backstairs influence.                  --Burke.

         Female caprice and backstairs influence. --Trevelyan.

Source : WordNet®

backstair
     adj : secret and sly or sordid; "backstairs gossip"; "his low
           backstairs cunning"- A.L.Guerard; "backstairs
           intimacies"; "furtive behavior" [syn: {backstairs}, {furtive}]