[Quiet Man Cometh]

16. What is the reason behind the phrase 'mad as a hatter'?

There are a few possible explanations for the phrase. I am most familiar with the thought that because the hat makers would often work with dyes or other chemicals that were toxic, the long term exposure could make them a little loopy. Mercury, for example. This explanation assumes the definition of "mad" as unhinged or crazy, as opposed to being synonymous with "angry."