What is the origin of the idiom "bats in the belfry" and when to use it?

The idiom "bats in the belfry" originated in the United States in the late 19th century or early 20th century. It alludes to the image of bats roosting in the belfry (the tower of a church where bells are hung), which was often seen as an odd or crazy behavior for the bats. Over time, the idiom came to be used metaphorically to describe someone whose thoughts or behavior are considered eccentric, irrational, or mentally unstable.