bats in one’s belfry, to have

To be slightly crazy or quite eccentric. The term alludes to the bat’s seemingly erratic flight in the dark, which is trans- ferred to thoughts flying about in the head. In reality, the bat has a sophisti- cated sonar system whose nature came to light only recently. In flight it keeps up a constant twittering noise that bounces back from solid objects in its path. This echo enables the animal to avoid actually bumping into obstacles. Never- theless, bats have long been associated with craziness. See also BLIND AS A BAT.