{ Backstairs, Backstair }, 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]