Deadline is currently almost exclusively used to mean a time by which a task must be accomplished, but this was not always so. In the past, deadline had a variety of meanings, all related to a boundary for which there was a severe penalty for crossing.
The oldest of these uses dates to the American Civil War and refers to a line drawn around a military prison outside of which a prisoner could be shot, a literal “dead” line. From the Congressional Record of 12 January 1864:
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The “dead line,” beyond which the prisoners are not allowed to pass.
Copyright 1997-2015, by David Wilton