{ }, 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 ]

. pos>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 ]