Green’s Dictionary of Slang

(US) a complete fool.

1991		D. Simon Homicide (1993) 239: He let some other brain-dead talk him into killing a woman.
2003		Simon & Burns ‘Ebb Tide’ Wire ser. 2 ep. 1 [TV script] Talking about the brain-deads in my Cane Street case.
2007		Skins ser.1 ep.1 [TV script] No, I don’t watch television, you braindeads.
2009		P. Temple Truth 332: Twenty minutes later he had his back against a wall in a lane off Sydney Road, listening to two braindeads come out of the roof.