the time by which something must be finished or submitted; the latest time for finishing something: a five o'clock deadline.
a line or limit that must not be passed.
(formerly) a boundary around a military prison beyond which a prisoner could not venture without risk of being shot by the guards.
Origin of deadline
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
a time limit for any activity
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
"time limit," 1920, American English newspaper jargon, from dead (adj.) + line (n.). Perhaps influenced by earlier use (1864) to mean the "do-not-cross" line in Civil War prisons, which figured in the Wirz trial.
And he, the said Wirz, still wickedly pursuing his evil purpose, did establish and cause to be designated within the prison enclosure containing said prisoners a "dead line," being a line around the inner face of the stockade or wall enclosing said prison and about twenty feet distant from and within said stockade; and so established said dead line, which was in many places an imaginary line, in many other places marked by insecure and shifting strips of [boards nailed] upon the tops of small and insecure stakes or posts, he, the said Wirz, instructed the prison guard stationed around the top of said stockade to fire upon and kill any of the prisoners aforesaid who might touch, fall upon, pass over or under [or] across the said "dead line" .... ["Trial of Henry Wirz," Report of the Secretary of War, Oct. 31, 1865]
Online Etymology Dictionary, © 2010 Douglas Harper