thats old hat

Thats old hat is a phrase which sounds to be used in incorrect English, but is actually exactly how this idiom is used. What an old hat is referring to is anything that is not new or modern any longer and has become outdated. For instance if someone refers to a certain trend or style that was modern in the 1980s, in the 2000s it is no longer modern and is quite outdated; this is known as old hat.