brain-dead, adj. — Green’s Dictionary of Slang

brain-dead adj.

[SE brain-dead, used of one who, while still technically alive, is in a persistent vegetative state]

utterly stupid, completely inept.

1989		P. Munro Sl. U.
1993		M.B. ‘Chopper’ Read How to Shoot Friends 165: It was the sort of brain dead plan that General Custer once hatched, and it didn’t do him any good either.
2001		N. Barlay Hooky Gear 254: You braindead bastard arsehole fuckwit.