Touched In The Head Meaning

(idiomatic) Demented; slightly mentally deficient.

Example: 1990, Reinaldo Arenas, The Palace of the White Skunks, page 136:
  I believe he's more touched in the head than I am.
1991 [1979], Valentin Rasputin, Antonina W. Bouis (translator), Farewell to Matyora, page 199:
  Who could say, perhaps Nastasya was even more touched in the head now, and if she used to make up stories about the old man crying and bleeding when she was here, maybe she had moved on to death over there?
2003, Tracee A. Hanna, A Little Bit of Sinning, page 163:
  It is broad daylight and if she can not simply guide the car along the highway then this bitch was more touched in the head that[sic] I thought.