How people divided time and set limits around it.
deadline (n.)
Collection
The Hours We Invented
A deadline is the latest time by which something must be done.
During the American Civil War, dead line was used for a boundary inside some military prisons. Prisoners were forbidden to cross it, and guards could fire on those who did. Andersonville is the best-known example, but the National Park Service notes that such lines were used at several prisons.1
The modern sense of a final time limit appeared in American newspaper jargon by 1919. The older prison use may have influenced it, but the connection is not certain.2
The deadline is Friday.