1. You must have bats in your belfry if you think your parents will let you see that movie.
2. Don’t listen to her. She doesn’t know what she’s talking about. She has bats in her belfry.

Synonym: have a screw loose , out to lunch , off one’s rocker. A belfry is the tower of a church where the bell hangs, and is analogous to one’s head. If a person has room for bats in his head, his head is full of space rather than brains. 5