1
general::
Phrase(s): soft in the head
Fig. stupid; witless. • George is just soft in the head. He’ll never get away with his little plan. • You’re soft in the head if you think I’ll go along with that.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Mentally deficient; also, silly, foolish. For example, He's nice enough but a bit soft in the head. The soft in this idiom, first recorded in 1775, alludes to a weakness in mental capacity.
American Heritage Idioms