'backwall' is a compound of 'back' and 'wall'. 'back' originates from Old English, specifically the word 'bæc', where it meant 'back' or 'rear'. 'wall' originates from Old English, specifically the word 'weall' (with connections to Proto-Germanic '*wallaz' and Latin 'vallum'), where it meant 'a rampart or wall'.

'back' and 'wall' were used separately in Old and Middle English; the phrase 'back wall' (two words) appears in Middle and Early Modern English. In technical or specialized contexts it has sometimes been written as 'back-wall' and later as the single-word form 'backwall' in modern usage.

Initially, the components meant 'rear' and 'rampart/wall', and when combined they meant simply 'rear wall'; this basic meaning has remained largely unchanged into modern usage.