Green’s Dictionary of Slang

shit-for-brains adj.

[shit for brains phr.]

stupid, unintelligent; a general abusive epithet.

[US]S. King Christine 344: If he could have been sorry for anyone, it would have been for the little shit-for-brains freshman.
[US]L. Heinemann Paco’s Story (1987) 161: Carry a rucksack — you look too bummy otherwise, like some slack-assed, shit-for-brains hippie.
[UK]J. Mowry Six Out Seven (1994) 418: Corbitt glanced up at Fluffy’s cage, wondering what sort of people shit-for-brains dogs liked.
[US]E. Bunker Mr Blue 311: We’ll teach you to run, fuckhead . . . shit for brains bastard.
[UK]J.J. Connolly Layer Cake 274: Maybe you think I’m just a fuckin gundog too, a shit-for-brains Mick.