A deadline is an absolute limit, usually a time limit, and was popularized by the newspaper business, in which getting stories written and printed on time is of ultimate importance.
But the expression comes from American Civil War prisoners, who were kept within crude makeshift boundaries, often just a line scratched in the dirt or an easily breached rail fence.
They were told, “If you cross this line, you are dead,” and soon the guards and prisoners simply called it what it was: a deadline.
These days, a restricted limit or time limit is often called a “deadline”.
