Green’s Dictionary of Slang

(US) lifelessly, listlessly.

1960		(con. 1940s) D. MacCuish Do Not Go Gentle (1962) 166: [If] ya move out on patrol [...] and go shufflin’ along dead-assed I’ll cut ya up good. Got that?
1970		J. Bouton Ball Four 355: [T]he phone was ringing in the bullpen. It was Harry telling us not to sit dead-assed out there in the bullpen.