You use the expression “Bats in the belfry” to indicate that someone is acting eccentric or crazy.
Example in use: ‘Wanda has bats in the belfry; she seems to think that the mailman is trying to steal her cat!’
Bats In The Belfry
Interesting fact
Although the phrase “Bats in the Belfry” has a very old-fashioned ring to it, the term originated in late 19th or early 20th century in America. A very early example comes from an October 1900 edition of an Ohio newspaper called The Newark Daily Advocate: “To his hundreds of friends and acquaintances in Newark, these puerile and senseless attacks on Hon. John W. Casssingham are akin to the vaporings of the fellow with a large flock of bats in his belfry.”