'backache' originates from an English compound of the words 'back' + 'ache', where 'back' comes from Old English 'bæc' meaning 'back' and 'ache' comes from Old English 'æce' meaning 'pain'.

'back' changed from Old English 'bæc' to Middle English 'bak' and then to modern English 'back'; 'ache' comes from Old English 'æce' (related to Proto-Germanic *ak- meaning 'pain'); the compound 'backache' developed in English to denote pain in the back.

Initially it simply referred to pain in the back, and over time it has retained that core meaning as 'pain in the back' in modern usage.