Green’s Dictionary of Slang

stupid, unintelligent; a general abusive epithet.

1983		S. King Christine 344: If he could have been sorry for anyone, it would have been for the little shit-for-brains freshman.
1986		L. Heinemann Paco’s Story (1987) 161: Carry a rucksack — you look too bummy otherwise, like some slack-assed, shit-for-brains hippie.
1993		J. Mowry Six Out Seven (1994) 418: Corbitt glanced up at Fluffy’s cage, wondering what sort of people shit-for-brains dogs liked.
1999		E. Bunker Mr Blue 311: We’ll teach you to run, fuckhead . . . shit for brains bastard.
2000		J.J. Connolly Layer Cake 274: Maybe you think I’m just a fuckin gundog too, a shit-for-brains Mick.
2004		R.G. Barrett Rosa Marie’s Baby (2013) [ebook] ‘A greater bunch of no-talent, shit-for-brains adolescents with attitude I’ve yet to come across’.