Originally a line around Civil War prison camps — cross it and you're shot dead. The Andersonville camp's 'dead line' was a literal execution boundary. Journalism adopted it in the 1920s for the latest time to submit a story. Your work deadline is a metaphorical firing line.
The latest time or date by which something should be completed; originally, a line that prisoners were forbidden to cross.
The term 'dead line' appears in Civil War-era records, most famously associated with the Andersonville prisoner-of-war camp in Georgia, where a line was marked around the prison stockade's inner perimeter — any prisoner crossing it would be shot. Whether Andersonville was the specific origin or merely the most documented instance is unclear; the term may have been used at other prisons too. The figurative sense of 'a time limit' emerged in American journalism by the 1920s, referring to the latest time a story