Just read an article about the expression and its origin: “As mad as a hatter”. The poor sould back then, they didn’t know any better.
It’s not about the Brexit, although sometimes you wonder what professions some of them have.
Anyone who knows the full story?
Being a Brit, I can answer this one. The Mad Hatter is a character in Alice in Wonderland by Lewis Carroll. But I believe that character cam as a result of hatters often going mad because they used mercury in their profession and that is not great for the brain.
It might be that Mad as a hatter - Wikipedia tells about all that can be known about the expression. Alice in Wonderland is full of things based on similar at the time well-known idioms (March Hare, little Bill, etc)
Of course, you also get the expression as mad as a March hare. If you live somewhere that has hares, as I do, I can attest they do indeed appear a bit mad in spring. A bit like puppies.