The term “as mad as a hatter” might conjure up images of the tea party from Lewis Carroll’s Alice's Adventures in Wonderland. Other recorded usages of the idiom "as mad as a hatter" actually predate Carroll's usage by at least 30 years. In the early to mid-1800s, it was used by writers in England, Scotland and Canada to describe anger or craziness. The other possible meaning of this term could be derived from accounts of mad hatter's syndrome.
At one time, the manufacturers of men's felt hats used camel’s hair or the pelts of beavers and other animals to make felt. Although beaver fur was easy to remove from the pelt, beaver pelts were in short supply and were expensive. The pelts of other animals, and camel hair, were traditionally prepared with liquids before scraping.
In Egypt, hat makers used camel urine for this process, and, in France, many workers used their own urine. The legend goes that the one hat maker’s urine was superior to most. It was discovered that he was being treated with mercury for syphilis.
No matter whether the legend is true, the use of mercury for preparing pelts became widespread. The fur was treated with mercury, scraped off the pelts and boiled in large vats. After that, the felt was steamed and made into hats. These processes took place in poorly ventilated factory rooms, and the workers were continually exposed to mercury fumes.
Mercury poisoning affects the mind as well as the body. Victims have little control over their movements and might shake uncontrollably, drool and have trouble speaking. They are subject to mood swings, irritability and episodes of anger. "Mad hatter’s syndrome" was a term used to define this condition; calling someone "as mad as a hatter'" would therefore imply erratic behavior.
Although Carroll might have had hatter’s syndrome in mind as he was writing his book, there are several other possible reasons why he created his two mad characters, the Mad Hatter and the March Hare. The character of the Hatter might have been inspired by a local eccentric named Theophilus Carter. Carter was a furniture maker who was known for his eccentric behavior and his top hat. He also was an amateur inventor who designed an alarm clock bed. This bed would tip the occupant out at the time for which the alarm was set.
The term "as mad as a hatter" also might be an extension of one of the animal similes, or comparisons, that have long been part of the English language, such as "as mad as a wet hen," "as mad as a cut snake" or "as mad as a march hare." The use of the word "mad" might mean refer to being angry, as a wet hen would be, or being crazy, as the buck rabbit in springtime is thought to be. It also has been suggested that this idiom might refer to an adder, a type of venomous snake. The phrase "as mad as a hatter" could then be considered to mean "as mad as an adder."