This phrase is used in conversation to suggest (lightheartedly) that someone is quite crazy. A similar expression is “mad as a March hare.”
Lewis Carroll’s ‘Hatter’ character from Alice’s Adventures in Wonderland, 1865, is our best-known mad hatter. The Hatter is not actually described as mad in the story — merely a participant at ‘a mad tea-party.’ But he’s not exactly sane, and he is portrayed as mad by the Cheshire Cat:
‘In that direction,’ the Cat said, ‘lives a hatter; and in that direction, lives a March Hare. Visit either you like: they’re both mad.’
Carroll may have taken his inspiration from the behavior of hatters and also from Theophilus Carter, who was an Oxford cabinet maker and furniture dealer with a reputation for eccentric behavior. In Carter’s case, the top hat certainly fits. He was something of a ‘mad inventor’ and came up with the alarm-clock bed, which woke people by tipping the bed over. Carroll would have been familiar with the sight of Carter, top hat and all, outside his shop in Oxford, where he lived in the 1850s, during the time that Carroll was an Oxford don.
“Mad as a March hare” describes the behavior of hares during breeding season, when they run and leap about the fields. But there are several theories for the origin of “mad as a hatter.”
The most popular theory suggests that the phrase was connected to mercury poisoning experienced by hat-makers as a result of the long-term use of mercury products in the hat-making trade. 19th-century hatters in Danbury, Connecticut, developed a condition known colloquially as the Danbury Shakes. The condition was characterized by slurred speech, tremors, stumbling, and, in extreme cases, hallucinations.
A neurotoxicologist states, “Mercury exposure can cause aggressiveness, mood swings, and anti-social behavior. It is therefore likely that the mercury in hat-making did lead to ‘mad’ hatters both in terms of rationality and plain old grumpiness.”
Another suggests that it arose from Roger Crab, a 17th-century eccentric who, after working for a short time as a hatter, gave all his goods to the poor and wore homemade sackcloth clothes.
Looking further back, we find the Anglo-Saxon word “atter” meaning poison, closely related to the word “adder” for the poisonous snake. Lexicographers William and Mary Morris in Morris Dictionary of Word and Phrase Origins (1977) favor this derivation because “mad as a hatter” was known before hat making was a recognized trade. According to A Dictionary of Common Fallacies (1980), ‘mad’ meant ‘venomous’ and ‘hatter’ is a corruption of ‘adder,’ or viper, so that the phrase ‘mad as an atter’ originally meant ‘as venomous as a viper.’
I like the Cheshire Cat’s take on it, that the hatter and the March Hare are both at least a little insane — in a pleasant way, of course.