Green’s Dictionary of Slang

1. a weak, ineffectual person, a coward.

1967		P. Thomas Down These Mean Streets (1970) 228: ‘Nothing but faggots and soft asses in there,’ he said.
1977		A. Bleasdale No More Sitting on the Old School Bench (1979) 63: You stay out of it, softarse.
1987		W.T. Vollmann You Bright and Risen Angels (1988) 210: He’s a softass out to screw up you and me and our ideals.
2012		D. Smith God in a Box 197: ‘Since when did you become a soft ass?’ ‘Is soft ass really the antonym of hard ass?’.

2. in attrib. use of sense 1.

1990		R.A. Spears Forbidden Amer. Eng. 174: She is such a soft-ass wimp.
2004		‘B.L.U.N.T.’ Betrayal of a Hustler 109: I see why you ain't never hit into no big time cake, you's a soft ass nigga!
2010		T. Fournier Becoming Tara n.p.: I feel like a soft-ass motherfucker. I’m supposed to be a thoroughbred and I let some pussy knock me to my knees.