'backache' originates from English, a compound of 'back' + 'ache' (both Old English in origin).

'back' comes from Old English 'bæc' and 'ache' from Old English 'æce'; the compound 'back-ache' appears in Middle/Late Middle English and later solidified as the modern single word 'backache'.

Initially it literally meant 'ache of the back'; over time it has retained that core meaning and is now the common everyday term for pain in the back.