'backstairs' originates from English, specifically a compound of 'back' and 'stairs', where 'back' meant 'rear' (Old English 'bæc') and 'stair(s)' referred to steps or a flight of steps.

'backstairs' developed from the literal phrase 'back stairs' in early Modern English and later became compounded; the figurative sense relating to secret or behind-the-scenes activity arose by analogy with activities carried out on or via the servants' stairs.

Initially it meant 'the stairs at the back of a house'; over time it acquired a figurative meaning of 'secret or underhand dealings/ influence' and this metaphorical sense is common in modern usage.