Green’s Dictionary of Slang

dead ass n.

[SE dead + -ass sfx/butt n.1 (1c)]
[1950s+] (US)

1. the seated rump, usu. as symbolic of laziness.

2. an idler, a lazy person.

3. one who is effectively dead.

4. a listless, de-energized person.

5. an utterly boring, useless person.