backlog (n.)

also back-log, 1680s, "large log placed at the back of a fire" to keep the blaze going and concentrate the heat; see back (adj.) + log (n.1). Figurative sense of "something stored up for later use" is first attested 1883, but this and the meaning "arrears of unfulfilled orders" (1932) might be from, or suggested by, log (n.2).