E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Private or unrecognised influence. It was customary to build royal palaces with a staircase for state visitors, and another for those who sought the sovereign upon private matters. If any one wanted a private interview with royalty, it was highly desirable to conciliate those
appointed to guard, the back stairs, as they could admit or exclude a visitor.