This definition is mostly used when someone is being really serious about something. But it can also be used when referring to a person who is not doing good in life. The term comes from dead (lazy, bum) ass (donkey) this term is used in ganglish.
Man 1: I got fired from my job and have been lazing around my house for months!
formerly dead-s meaning dead serious. the ghetto form of dead serious is dead-s, which is not used anymore. less stress is used wen saying the word dead-ass so it has replaced dead-s.