There are a number of expressions with "dead" as an unlikely modifier:

dead certain
dead eye
dead giveaway
dead heat
dead last
dead serious
dead set
dead to rights
dead weight

Why so dead?

(Other expressions, where "dead" itself is modified, as in "dead as a door nail," do not seem anomalous in the same way.)

A lot of those idioms use 'dead' in the sense of 'certain, sure, exact'. I would think that use arose because if there's one thing we can be certain of, it's death (and taxes).

"Are you sure?"
"I'm tax serious!"

"How did you know, detective?"
"That clue was a tax giveaway."

"How did the race end?"
"It was a tax heat"

quote:

dead eye

Seeing that for the first time, a person might assume it meant "blind."

Seeing Proof's take on that set me looking, and here's what turned up:

http://en.wikipedia.org/wiki/Deadeye

In marksmanship, deadeye means one who is an excellent shot.

There's also dead reconing, the quick and the dead, and top/bottom dead center. I think it may well refer to the point of no motion in some cases.

I fired "expert" in the army and everyone called me "Deadeye Dick." Over the years the nickname lost first the "eye", then the "Dead."

I hear you, Robert. It is rather strange. At first I was going to say it means "serious," but that sure doesn't fit them all...for example, deadpan.

"Deadpan" just uses the "dead" meaning; "pan" mean face of course.

quote:

At first I was going to say it means "serious," but that sure doesn't fit them all...for example, deadpan.

I think the "serious" interpretation fits for deadpan. The deadpan face is a serious face in the presence of humor, right? Dead center is seriously in the center. Dead eye is a seriously good aim.

Or maybe it's really more like "exactly".