'backhouse' originates from Middle English, specifically the compound formed from 'back' + 'house', where 'back' meant 'rear' and 'house' meant 'building or dwelling'.

'backhouse' changed from Middle English forms such as 'bakhous' or 'bak(h)ous', ultimately deriving from Old English elements 'bæc' and 'hūs', and eventually became the modern English word 'backhouse'.

Initially, it meant 'a building or part of a building at the rear (often used as a privy or outbuilding)'; over time it retained the sense of 'rear outbuilding, especially an outhouse' but has become archaic or dialectal in modern usage.