A backlog is a pile of work that is still waiting — and often it should have been finished already. Unanswered email, unsent orders, patients still waiting for a scan, court cases that have not been heard: those can all be a backlog. The picture is of things stacked up behind you. Offices, hospitals, shops, and software teams all use the word; in tech offices people sometimes just keep the English word. It can be a verb too: work backlogs when there are not enough people, or a team backlogs tasks into a waiting list. It is a normal grown-up word, a little formal but not stiff. Though actually, the first meaning, back in the 1680s, was a large log at the back of a fire, the slow-burning one that kept the heat. In the 1880s people started using it for anything stored up for later use, and from there it became “work that has piled up.” The written-record sense of log (as in a logbook) may have coloured it too — but that is only a maybe, not something to treat as fact.