Mad as a Hatter

Mercurial disease, caused by chronic mercury poisoning, was common among hatmakers, and the phrase "mad as a hatter" was in use three decades before "Alice's Adventures in Wonderland" was published. 

Due to an occupational hazard caused by a process called "carroting," part of felt hatmaking, hatters often developed symptoms such as tremors, excessive saliva, forgetfulness and unprovoked anger.