This phrase was originally used with reference to passages in the biblical epistles in which St. Paul compares the life-giving spirit of the New Testament with what he sees as the dead letter of the Mosaic law. Later (until the late 19th century) Dead-letter Office was the name given to the organization that dealt with unclaimed mail or mail that could not be delivered for any reason. The expression has been used metaphorically for an obsolete or unobserved law since the mid 17th century.
1998 - Spectator - They were saying on the news that some provision of the Stormont agreement might end up a dead letter.
RELATED IDIOMS :
dead and buried
used to emphasize that something is finally and irrevocably in the past
dead as a dodo = dead as the dodo
no longer alive
no longer effective, valid or interesting - Informal
The name dodo comes from Portuguese duodo meaning simpleton. It was applied to the large flightless bird of Mauritius because the bird had no fear of man and so was easily killed, being quickly wiped out by visiting European sailors. The dodo's fate has made it proverbial for something that is long dead and the name has been used metaphorically for an old-fashioned, stupid or unenlightened person since the 19th century.
2000 - John Caughie - Television Drama - The once pleasant family hour is now as dead as a dodo.
dead as a doornail = dead as mutton
A doornail was one of the large iron studs formerly often used on doors for ornamentation or for added strength. The word occurred in various alliterative phrases (e.g. deaf as a doornail and dour as a doornail) but dead as a doornail is now the only one in common use.
a dead cat bounce
a misleading sign of vitality in something that is really moribund – informal
A dead cat might bounce if it is dropped from a great height. The fact of it bouncing does not reliably indicate that the cat is alive after all. The expression was coined in the late 20th century by Wall Street traders to refer to a situation in which a stock or company on a long-term, irrevocable downward trend suddenly shows a small temporary improvement.
dead from the neck up = dead from the chin up
stupid - Informal
1990 - Film Comment - Steward subscribes to the notion that all women are nitwits and lunkheads, dead from the neck up.
dead in the water
unable to function effectively
Dead in the water was originally used of a ship and in this context means unable to move.
1997 - Times - And Oasis? Well, they are hardly dead in the water, having sold three million copies of BE HERE NOW.
in serious trouble – informal
1989 - Tracy Kidder - Among Schoolchildren - You're dead meat, I'm going to get you after school.
dead men's shoes
property or a position coveted by a prospective successor but available only on a person’s death
the dead of night
the quietest, darkest part of the night
the dead of winter
the coldest part of winter
The sense of dead here and in the previous idiom developed in the 16th century from dead time of______ meaning the period most characterized by lack of signs of life or activity.
dead on your feet
extremely tired – informal
This expression was a development from the phrase dead tired as an exaggerated way of expressing a feeling of exhaustion. Dead is sometimes also used on its own to mean exhausted.
dead to the world
unconscious – informal
2000 - Michael Ondaatje - Anil's Ghost - The nurse tried to wake him. But he was dead to the world.
from the dead
from a state of death
from a period of obscurity or inactivity
make a dead set at
make a determined attempt to win the affections of - British
Dating from the early 19th century, this was originally a sporting idiom referring to the manner in which a dog such as a setter or pointer stands stock still with its muzzle pointing in the direction of game.
over my dead body
used to emphasize that you completely oppose something and would do anything to prevent it from happening - informal
would not be seen dead in = would not be seen dead with = would not be seen dead at = would not be caught dead in = would not be caught dead with = would not be caught dead at
used to express strong dislike or disinclination for a particular thing or situation – informal
1997 - Independent - Kate's books, said one literary editor, can be read happily by those who wouldn't be seen dead with a Catherine Cookson.