When you have extra work to do that keeps accumulating, you have a backlog. Sometimes people stay late at their jobs to take care of a backlog.
A backlog of tasks might slow down an ordinary day at an office or store, or require workers to come in on the weekend to catch up. In 1680, a backlog meant "a large log placed at the back of a fireplace," and by the 1880s it came to also mean "something stored for later use." It wasn't until the early 20th century that a backlog most often referred to "extra unfulfilled orders."