have rocks in (one's) head
To be thoroughly stupid, crazy, absurd, incorrect, etc. You must have rocks in your head if you think the bank will approve you for another loan.
have rocks in one's head
Fig. to be silly or crazy. John is a real nut. He has rocks in his head. I don't have rocks in my head—I'm just different.
rocks in one's head, have
Show poor judgment, act stupidly, as in If you think that's an accurate summary, you've got rocks in your head. The rocks in this idiom are a supposed substitute for brains. [Slang; 1940s]