Be/go soft in the head informal verb phrase
To become stupid, foolish, silly or crazy
I think you must be soft in the head when you decide to invest your money in that company.
Jack is a good student, but he seems to be soft in the head.
I suddenly notice a man walking on the street. He must be soft in the head when he talks to himself and looks at the sky.
What are you doing with my computer, John? Are you soft in the head?.
To have no money
Used to describe someone who is naive, gullible, inexperienced, easily fooled, ignorant, unsophisticated, etc.
To say, or believe something or someone that seems completely crazy, delusional, or stupid
Stupid or crazy
The verb "be/go" should be conjugated according to its tense.
This idiom dated back to 1775, references to a mental capacity problem.