'backfill' originates from English, specifically the compound of the words 'back' and 'fill', where 'back' meant 'to the rear' or 'backward' and 'fill' meant 'to make full'.

'backfill' developed as a compound in technical contexts (mining, construction) in the late 19th to early 20th century, combining 'back' + 'fill' to describe the action or material used to fill excavations; it then became widely used in civil engineering and later metaphorically in staffing and computing.

Initially, it referred primarily to the physical act or material used to refill excavations; over time the term extended metaphorically to mean filling vacant jobs or schedule gaps and other non-physical 'filling' actions.