deadline
The fencepost surrounding Andersonville Prison marked the "dealine," beyond which anyone would be shot.

Idiom: Meet a deadline

Definition: To finish a task by a pre-determined time.

Example: "A deadline is negative inspiration. Still, it's better than no inspiration at all." – Rita Mae Brown

Origin: While the word "deadline" may have been used to describe lines that did not move, such as one used in angling, most etymologists seem to agree that the current usage of the word dates back to the American Civil War.

At that time, a deadline was a line that was marked around the perimeter of a military prison. Soldiers guarding the prison were authorized to shoot anyone who crossed this line. 

However, a later usage may be responsible for our current use of the word deadline to mean  a set time by which a job needs to be completed. In the early 19th century, the word was used to describe a line on printing presses, beyond which text would not print properly. 

It's unclear which of these uses morphed into our modern usage, but the printing press term seems more likely to us, since the modern sense of the word first became common in the publishing world.

BONUS IDIOM:
One of our readers wrote to us recently to ask if we could find the origin of the saying, "That really frosts my bobber, meaning 'that really angers me.'"

"My grandfather always used that saying," writes Gordon Chinander, "and I have been curious about the origins."

We weren't able to find any specific information on the origins of this idiom, nor were we to find may examples of its usage. It's probably safe to assume that it originated with ice fishers. There is an annual ice fishing tournament in East Grand Forks, Minnesota called "The Frosty Bobber." However, we're not sure why a frosty bobber would be annoying, so maybe in this instance "bobber" refers to something else entirely.