Origin: During the Civil War (1861-65), the first deadline was literally a death line — a line that promised death. Outside the Confederate prison camp located in Andersonville, the guards drew a line in the ground about 17-20 feet from the prison fence. Prisoners were informed that if they crossed that line, they would be shot dead. The earliest recorded instance of the word was in an inspection report, dated May 10, 1864, by Confederate Captain Walter Bowie describing the dead line: “On the inside of the stockade and twenty feet from it there is a dead-line established, over which no prisoner is allowed to go, day or night, under penalty of being shot.” Over time the term was applied to situations that involved specific boundaries. In 1909, the legendary short story writer, O. Henry, referred to impropriety as “crossing the dead line of good behavior.” By the early 1920s, the two words were combined and newspaper editors borrowed the term “deadline” to refer to the specific day and time that a story was due, implying that a late story was effectively “dead” (or if the editor was very ill-tempered and violent, perhaps the writer would be dead as well). The word then migrated to the world of commerce. Almost two centuries later, just about everybody is deluged with deadlines on a daily basis.
For further reading: Word and Phrase Origins by Robert Hendrickson, Facts on File (2008)