Q:

Where does the phrase "meta means murder" come from?

When Meta is down, the now famous "Meta Means Murder" image is shown. Where does that phrase come from?

A:

Meta is Murder by Jeff Atwood.
This is likely the original source of the phrase and of the image. As Shmork writes in a comment:

I just spent 10 minutes looking for a "META IS MURDER" T-shirt before I realized that you had modified a "MEAT IS MURDER" one. D'oh.

