Mad As a Hatter (Origin)
What Is the Origin of the Saying "Mad As a Hatter"?To be as mad as a hatter means to be crazy or to behave unpredictably.
The term comes from the hat-making industry in the middle ages, which used the highly toxic mercury nitrate in the hat’s rim. The people who made these hats would often suffer poisoning which would make them act oddly. Therefore, many hatters were assumed to be crazy.