Definition from Wiktionary, the free dictionary
See also: back stairs
backstairs pl (plural only)
- A staircase at the rear of a building or one normally only used by servants and tradesmen.
- An indirect or furtive means of access or intercourse.
1934 March 27, “De Valera Move Hits Dignity Of Governor-General”, in Christian Science Monitor:
- The straight-forward course, they say, would be to proclaim outright, instead of trying to bring it in by the backstairs.
- Secret or furtive.
- 1770, Edmund Burke, Thoughts on the Cause of the Present Discontents, 
- […] if some peers (I am very sorry they are not as many as they ought to be) set themselves, in the great concern of peers and commons, against a back-stairs influence and clandestine government, then the alarm begins; then the constitution is in danger of being forced into an aristocracy.
- 1880, George Otto Trevelyan, The Early History of Charles James Fox, London: Longmans, Green, and Co., 1881, Chapter 8, p. 364, 
- The danger of the situation was increased by the mischievous conduct of Alderman Townshend, who had been brought down to the House, pale and bandaged from a recent surgical operation, in order to pour forth a diatribe against female caprice and backstairs influence; […]
- 2004, Alan Hollinghurst, The Line of Beauty, Bloomsbury, 2005, Chapter 3,
- This backstairs visit was all about sex […]
- 1770, Edmund Burke, Thoughts on the Cause of the Present Discontents,