When something is old hat, it has been ❛seen and done before and is no longer new or interesting❜. Look at how this English idiom is used in the following examples:
After a few years, Sofian’s job became old hat.
The song might seem old hat to some people, but Fred’s rendition of ❛My Way’ is completely fresh and new.
Unfortunately, most of that comedian’s ❛knock knock❜ jokes were old hat.
A job, a song or a joke, especially a ❛knock knock❜ joke, can definitely be old hat, i.e. old, routine and uninteresting. A ❛knock knock❜ joke is a joke like the following:
A. Knock Knock.
B. Who’s there?
B. Boo who?
A. No need to cry. It’s only a joke.
Did you laugh? Probably not. The fact of the matter is that, while adults tend to find ❛knock knock❜ jokes old hat, young children, it seems, cannot get enough of them. Have a look at the following video: