Green’s Dictionary of Slang

stupid, brainless.

1976		E. Thompson Caldo Largo (1980) 272: Here! Over here! Look over here, you shitbrained fuckers!
1991		B. Hamper Rivethead (1992) 69: Many is the time I felt I was going to go stark raving shit-brained wacko.
1993		I. Welsh Trainspotting 178: This one’s for you [...] Rambo, or Tyson or whatever the fuck your shite-brained fuck-wit of an owner has dubbed you.
2012		M. Lacher On the Bro’d 130: [A] completely shitbrained idea idea that watching your friend bone your lady would somehow be awesome.