Dictionary:

brainsick

  (brān'sĭk')

Of, relating to, or induced by a mental disorder; insane or mad.

adjective

Afflicted with or exhibiting irrationality and mental unsoundness: crazy, daft, demented, disordered, distraught, dotty, insane, lunatic, mad, maniac, maniacal, mentally ill, moonstruck, off, touched, unbalanced, unsound, wrong. Informal bonkers, cracked, daffy, gaga, loony. Slang bananas, batty, buggy, cuckoo, fruity, loco, nuts, nutty, screwy, wacky. Chiefly British crackers. Law non compos mentis. Idioms: around the bend, crazy as a loon, mad as a hatter, not all there, nutty as a fruitcake, offout ofone's head, off one's rocker, of unsound mind, out of one's mind, sick in the head, stark raving mad. See sane/insane.

The adjective has one meaning:

Meaning #1: affected with madness or insanity
  Synonyms: crazy, demented, distracted, disturbed, mad, sick, unbalanced, unhinged