The word deadline in the sense that we know it today first emerged in early twentieth century newspaper slang. It's thought to have originated from a Civil War term (frequently stylized with a hyphen) referring to an actual physical boundary in prisoner-of-war camps that inmates were not allowed to cross, on penalty of death. The term was popularized by its use in the highly-publicized trial of Confederate general Henry Wirz, who infamously established such a line at his squalid camp in Andersonville, Georgia. Later on, apparently people metaphorically extended the word to time limits on the sense that submitting something late is like getting shot. One piece of evidence that would challenge this theory, however, is that there are attestations of the noun deadline being used in the 1920s to refer to a marking on the bed of a printing press. Maybe that's also related, though - it's uncertain.
Hello! I'm Adam Aleksic. This year, I graduated from Harvard University with a degree in Government and Linguistics. There, I co-founded the Harvard Undergraduate Linguistics Society and wrote a thesis on Serbo-Croatian language policy, magna cum laude. In addition to etymology, I also really enjoy philosophy, trivia, vexillology, geography, board games, conlanging, art history, and law.