The term deadline originated from prison camps during war, and referred to a physical line or boundary.[1] Guards would shoot any prisoner who crossed the deadline.[2] The "deadline" in such prison camps as Confederate Prison Camp Andersonville or Union Prison Camp Douglas Chicago during the Civil War was "Inside (the wall), about 19 feet from the wall,” which prisoners were forbidden to cross. The "deadline" was intended to prevent prisoners from climbing over the stockade or from tunneling under it.[3] The term was later adapted in its use to time lines, perhaps to show the seriousness of an end date in a timeline by referring to it as a "deadline."

@Curionic

#staycurious

Source