'backlog' originates from English, composed of 'back' + 'log', where 'back' meant 'rear' and 'log' meant 'a piece of wood or timber used in a fireplace or as a record.'

'backlog' was originally used literally for a log placed at the back of a hearth to keep a fire going; later it came to be used figuratively for a reserve supply or an accumulation, and eventually for accumulated unfinished work—leading to modern senses such as 'an accumulation of uncompleted tasks.'

Initially it meant 'a log at the back of a hearth' or 'reserve of logs'; over time it evolved into the current meaning of 'an accumulation of unfinished work or pending items.'