A staircase at the rear of a building or one normally only used by servants and tradesmen.
1905, Ernest William Hornung, “The Spoils of Sacrilege” in A Thief in the Night,
Other feet were already in the lower flight of the backstairs; but the upper flight was the one for me, and in an instant we were racing along the upper corridor […]
An indirect or furtive means of access or intercourse.