"Mad as a hatter" is a colloquial English phrase used in conversation to suggest (lightheartedly) that a person is suffering from insanity. The etymology of the phrase is uncertain, with explanations both connected and unconnected to the trade ofhat-making. The earliest known appearance of the phrase in print is in an 1829 issue ofBlackwood's Edinburgh Magazine, predating theHatter fromLewis Carroll'sAlice's Adventures in Wonderland by several decades.

There are many theories about the possible origin of the saying:

• Mercury poisoning of hat-makers – In 18th and 19th century England, mercury was used in the production offelt, which was commonly used in the hat-making trade at the time. Long-term use of mercury products often resulted inmercury poisoning-inducederethism among hat-makers.^[1][2] In the late 19th-century United States, a notable example occurred inDanbury, Connecticut, where hat making was a major industry. Instances of erethism were so widespread among hat-makers, the condition became known locally as the "Danbury Shakes." It was characterized by slurred speech, tremors, stumbling, and in extreme cases hallucinations.
• An incident ofnominalization of the verbhatter, which means "To harass; to weary; to wear out with fatigue," according toSamuel Johnson'sA Dictionary of the English Language, published in 1755. In the text, he cites a passage from the work ofJohn Dryden as an example of usage: "He'shatter'd out with pennance."^[3]
• Roger Crab, a 17th-century hermit who, after working for a short time as a hatter, gave all his goods to the poor and wore homemade sackcloth clothes.^[1] However, this was presaged by political and religious radicalism, and was followed by a long married life.^[4]
• An adaptation of theOld English wordatter meaning "poison", and closely related to the wordadder for the venomouscrossed viper. Lexicographers William and Mary Morris inMorris Dictionary of Word and Phrase Origins (1977) favour this derivation because "mad as a hatter" was known before hat making was a recognized trade.^[1] According toA 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'."

Boston Corbett, who shotAbraham Lincoln's assassinJohn Wilkes Booth, spent his early life as a hat maker. It is believed that the effects of his early life job affected his decision-making for his future. He was considered "mad as a hatter" for going against orders when his unit had Booth surrounded in a barn inVirginia, and Sergeant Corbett shot Booth instead of taking him alive. After investigation, Sergeant Corbett was forgiven for his disobedience, but left the Army and went back to hat making. After a few years, Corbett suffered further mental illness, and he was thrown into an insane asylum. Corbett managed to escape, and he was never seen again.^[5]

In a section of Volume 25 ofBlackwood's Edinburgh Magazine, headed Noctes Ambrocianæ. No. XLIV, there is a conversation between a group of fictional characters:

NORTH: Many years – I wasSultan of Bello for a long period, until dethroned by an act of the grossest injustice; but I intend to expose the traitorous conspirators to the indignation of an outraged world.

TICKLER (aside to SHEPHERD.): He's raving.
SHEPHERD (to TICKLER.): Dementit.

ODOHERTY (to both.): Mad as a hatter. Hand me a segar.^[6][7][8]

Canadian authorThomas Chandler Haliburton used the phrase twice in his 1835 bookThe clockmaker; or the sayings and doings of Samuel Slick of Slickville: "And with that he turned right round, and sat down to his map and never said another word, lookin' as mad as a hatter the whole blessed time" and "Father he larfed out like any thing; I thought he would never stop – and sister Sall got right up and walked out of the room, as mad as a hatter. Says she, Sam, I do believe you are a born fool, I vow."^[6][9]

• The dictionary definition ofmad as a hatter at Wiktionary
• Mad as a March hare
• Going postal

• English phrases
• British English idioms
• Quotations from literature
• 1820s neologisms
• 1820s quotations
• Hatmaking
• Fiction about mental health

• Articles with short description
• Short description is different from Wikidata