Green’s Dictionary of Slang

1. emotionally exhausted.

1997

N. Barlay Curvy Lovebox 16: I reach Finsbury Square an’ crash on a bench brained.

2. drunk.

[1611		Shakespeare Tempest III i: trin.: If th’ other two be brained like us, the state totters. ste.: Drink, servant monster, when I bid thee: thy eyes are almost set in thy head].
2000		Indep. Rev. 4 Mar. 4: That alcohol is being obtained / By kids as young as I was then / Who spend their spare time getting brained.