"(as|) nutty as a fruitcake" Meghatározása és Jelentése

What is the origin of the idiom "nutty as a fruitcake" and when to use it?

The phrase "nutty as a fruitcake" has emerged in the United States in the 20th century. This idiom is used to describe someone who acts strangely, eccentrically, or in a bizarre manner.