When I first heard the idiom "old hat" it was used as if everyone knows that it means uninteresting, well-trodden, and boring.
The phrase fascinated me then. It has since become old hat.
We all carry in ourselves knowledge, experiences, and ways of seeing the world. Often we forget that others do not share these.
What we see as old hat, is fascinating and new to other people.
One of the greatest joys in life is uncovering other's hats and trying it on for ourselves.
Another great joy is to identify and share your old hats with others.