Green’s Dictionary of Slang

1. (US) of a place, inactive, boring.

1971		P. Dennis Paradise 49: Little chance of being picked up in a dead-ass place like this.
1988		O. Hailey Kith and Kin 13: What’s there to do in this dead-ass town?
1997		(con. 1970s) G. Pelecanos King Suckerman (1998) 20: He sat [...] in a hot, treeless, dead-ass warehouse district in Southeast D.C.
2004		B. Ray Cold-Wing Dinner 187: I ain’t legit. No credit, no way to ever get out of this dead ass neighborhood.

2. (W.I.) a general negative intensifier.

1959		A. Salkey Quality of Violence (1978) 135: You is a blasted, dead ass liar!
1974		(con. 1940s) E. Thompson Tattoo (1977) 235: There’s this guy [...] who’s had a thing about her ever since they were little kids. Writes these deadass letters to her.
1989		C. Jennings Mouthful of Rocks 46: [H]e was fucked if he was going to let a group of dead-ass wankers spoil his time off.

3. (US) as adv., seriously.

2016

The Cut (NY Times Mag.) Mar. 🌐 Deadass — Seriously. As in: ‘That’s deadass a great Vine’.