The Collaborative International Dictionary of English v.0.48:

Backstairs \Back"stairs`\, Back stairs \Back" stairs`\
   (b[a^]k"st[^a]rz`) n.
   Stairs in the back part of a house, as distinguished from the
   front stairs; a second staircase at the rear of a building;
   hence, a private or indirect way.
   [1913 Webster + WordNet 1.5] Backstairs

The Collaborative International Dictionary of English v.0.48:

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

   Syn: clandestine, cloak-and-dagger, hugger-mugger, hush-hush,
        on the quiet(predicate), secret, subterranean,
        surreptitious, undercover, underground.
        [1913 Webster + WordNet 1.5]

              A backstairs influence.             --Burke.
        [1913 Webster]

              Female caprice and backstair influence.
                                                  --Trevelyan.
        [1913 Webster]

WordNet (r) 3.0 (2006):

backstairs
    adj 1: secret and sly or sordid; "backstairs gossip"; "his low
           backstairs cunning"- A.L.Guerard; "backstairs
           intimacies"; "furtive behavior" [syn: backstair,
           backstairs, furtive]
    n 1: a second staircase at the rear of a building

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "backstairs":
   back-door, clandestine, covert, feline, furtive, hidlings,
   hole-and-corner, hugger-mugger, privy, quiet, shifty, skulking,
   slinking, slinky, sly, sneaking, sneaky, stealthy, surreptitious,
   under-the-counter, under-the-table, undercover, underground,
   underhand, underhanded, unobtrusive