What is the origin of the idiom "old hat" and when to use it?

The idiom "old hat" likely originates from the practice of using hats as a fashion accessory. In the 19th century, people frequently wore hats as a symbol of style and social status. As fashions changed, what was once considered stylish and new would eventually become outdated or "old hat." This expression is now used to describe something that is no longer considered new, exciting, or innovative.