Green’s Dictionary of Slang

(US) a fool; thus as a term of address.

1960		(con. 1940s) D. MacCuish Do Not Go Gentle (1962) 216: Shut up, you dumb shit.
1961		R. Gover One Hundred Dollar Misunderstanding 42: He laugh too, dum she-it.
1967–8		Baker et al. CUSS 110: Dumb shit A person without much social or academic ability.
1972		D. Pendleton Boston Blitz (1974) 95: Books Figarone was no dumb shit.
1977		(con. 1969) M. Herr Dispatches 201: Well, dumb shit, what’d you think?
1989		Hackworth & Sherman About Face (1991) 91: We were just a bunch of dumbshits trying to articulate a comrade’s courage.
1991		B. Hamper Rivethead (1992) 42: I swear the dumbshit’s on some crazy drug trip.
2000		C. Cook Robbers (2001) 6: You dumbshit, said Eddie, look what you did.