Etymology of Phrases —> “As mad as a hatter”
In the 17th and 18th centuries, hat-makers (hatters) often had cognitive issues (or went mad) as a result of mercury poisoning, a side-effect of manufacturing felt hats. The famous Mad Hatter of Alice in Wonderland is also a play on this phenomenon.
Mark Bere Peterson’s Hauntings, Urban Legends, Paganism & Witchcraft 