To be slightly crazy or quite eccentric. The term alludes to the bat’s seemingly erratic flight in the dark, which is transferred to thoughts flying about in the head. In reality, the bat has a sophisticated 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. Nevertheless, bats have long been associated with craziness. See also blind as a bat.