The accepted story is that, in certain Confederate prison camps during the American Civil War, a line would be drawn around the prisoners' area. POWs who dared to pass beyond that line would risk being shot dead. Thus, the line came to be called the dead line.
Deadlines these days are moments in time, not in space.
A deadline is also the reason this post appears today and not yesterday, when I normally write about a word. Yesterday — well, all week, really — I was working against a deadline on a particularly frustrating and difficult freelance copy editing project. I finished it late last night but, truth be told, I did take a small step across that deadline.
So today, I am thankful not only that such an aggravating project is out of my hands, but that I haven't had me kneecaps shot off, either.