'backachy' originates from English, combining 'back' + 'ache' + the adjectival suffix '-y' (forming an adjective meaning 'having an ache').

'ache' comes from Middle English 'ache' (from Old English 'æce' meaning 'pain'), and the suffix '-y' has long been used to form adjectives (e.g., 'achy' from 'ache' + '-y'). 'Backachy' was formed in modern English by attaching this adjectival pattern to 'back'.

Initially the components referred simply to 'back' and 'pain'; over time the compounded adjective came to mean specifically 'having a backache' or 'causing back discomfort' in informal usage.