Brewer's: Back-stair Influence
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.
Once, we confess, beneath the patriot's cloak, From the cracked bag the dropping guineas broke, And, jingling down the back stairs, told the crew Old Cato is as great a rogue as you.
Pope: Epistle to Lord Bathurst, 35–8.