To be as mad as a hatter means to be completely insane or demented.
Also: Mad as a March hare
Origin of “Mad as a Hatter”
Contrary to popular belief, the idiom mad as a hatter did not originate with Lewis Carroll’s famous character in Alice’s Adventures in Wonderland (1865). The expression was already well-established in English speech during the early 1800s, decades before Carroll’s book was published.
The Medical Origin: Mercury Poisoning in Hatmaking
The expression stems from the historical occupation of felt hatmaking. During the 18th and 19th centuries, hatters used mercurous nitrate to process animal pelts and stabilize felt. Prolonged exposure to mercury vapor caused chronic mercury poisoning, a severe occupational illness known medically as erethism or erethism mercurialis, and colloquially as mad hatter disease.
This neurological damage caused severe tremors, extreme irritability, pathologically intense shyness, depression, and mood swings. In advanced cases, prolonged exposure led to slurred speech, hallucinations, and complete delirium.
Lewis Carroll and the Character Inspiration
While the character of the Mad Hatter in Alice in Wonderland exhibited eccentric and erratic behavior, it is unclear whether Carroll directly based the character on the medical condition. Many historians believe Carroll drew inspiration from Theophilus Carter, a well-known eccentric British furniture maker and inventor in Oxford who was nicknamed “The Mad Hatter” due to his unusual habits and propensity for wearing top hats.
Literary Whimsy vs. Medical Reality
In fact, Carroll’s character displayed almost none of the actual clinical symptoms associated with mad hatter disease. While real-world mercury poisoning left victims pathologically shy, anxious, withdrawn, and severely depressed, Carroll’s Hatter was boisterous, confrontational, and spent his days hosting a loud, never-ending tea party.
It is far more likely that Carroll simply latched onto the popular 19th-century phrase “mad as a hatter”, along with the concept of hatmaking, to create an eccentric, whimsical figure whose “madness” was purely nonsensical rather than symptomatic of a tragic occupational illness.
The Related Variant: “Mad as a March Hare”
Another popular variation mentioned alongside the Hatter is mad as a March hare. This phrase refers to the erratic leaping and boxing behavior of male hares during their spring breeding season (traditionally thought to peak in March).
Because Carroll included both a Mad Hatter and a March Hare at the famous tea party scene, he brought both existing 19th-century idioms together under one iconic literary roof.
Sentence Examples
“We never went near Mr. Blackwell’s house. He’s mad as a hatter.”
“My boss is as mad as a hatter. He told us all that he’d fire us if we didn’t buy him a birthday present.”
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Similar and Related Idioms
- Nutty as a Fruitcake: Completely foolish, eccentric, or insane. (Derived from the 1920s play on words where “nutty” meant crazy and fruitcakes literally contained lots of nuts.)
- Off His Rocker: Acting irrationally, crazy, or behaving in a completely absurd manner.
- Bats in the Belfry: Eccentric, crazy, or having wild ideas; having a mind filled with chaotic or odd thoughts.
- Off The Deep End: To go off the deep end (or simply be off the deep end) means to lose control of your emotions, act irrationally, or exhibit extreme anger, distress, or madness.
- Basket Case: Someone who is emotionally or mentally overwhelmed to the point of being unable to function.
- Go Ape (or Ape Shit): To become wildly excited, furious, or behave in an extremely uninhibited, aggressive, or chaotic way.