Mad as a hatter
"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 of hat-making.
The origin of the saying may derive from;
- Mercury poisoning of hat-makers – A popular explanation of the phrase suggests that it was connected to mercury poisoning or Korsakoff's syndrome 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. 
- An incidence of nominalization of the verb hatter, which means "To harass; to weary; to wear out with fatigue." according to Samuel Johnson's A Dictionary of the English Language published in 1755. In the text he cites a passage from the work of John Dryden as an example of usage: "He's hatter'd out with pennance."
- 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.
- An adaptation of the Anglo-Saxon word atter meaning poison, closely related to the word adder for the poisonous Crossed Viper. Lexicographers William and Mary Morris in Morris Dictionary of Word and Phrase Origins (1977) favour 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'."
In a section of the January–June 1829 issue of Blackwood's Edinburgh Magazine, headed Noctes Ambrocianæ. No. XL1V, there is a conversation between a group of fictional characters:
NORTH: Many years – I was Sultan 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.ODOHERTY (to both.): Mad as a hatter. Hand me a segar.
SHEPHERD (to TICKLER.): Dementit.
Canadian author Thomas Chandler Haliburton used the phrase twice in his 1835 book The 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."
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- Rees, Nigel (1987). Why Do We Say ...?. ISBN 0-7137-1944-3.
- Barbara Mikkelson (13 July 2007). "Mad As a Hatter". snopes.com. Retrieved 6 September 2009.
- Mercury Workshop, p. 23, Ohio Indoor Air Quality Coalition, 2008. "In the late 1800’s hat makers, or hatters, used to use mercury nitrate when working with beaver fur to make felt. Over time, the hatters started exhibiting apparent changes in personality and also experienced tremors or shaking. Mercury poisoning attacks the nervous system, causing drooling, hair loss, uncontrollable muscle twitching, a lurching gait, and difficulties in talking and thinking clearly. Stumbling about in a confused state with slurred speech and trembling hands, affected hatters were sometimes mistaken for drunks. The ailment became known as 'The Danbury Shakes' in the community of Danbury where hat making was a major industry. In very severe cases, they experienced hallucinations. The term “mad as a hatter” may be a product of mercury toxicity. The practice did not completely stop until 1943."
- Johnson, Samuel (2005). A Dictionary of the English Language: An Anthology. Penguin. p. 289. ISBN 0-14-144157-7.
- Gary Martin. "As mad as a hatter". phrases.org. Retrieved 6 September 2009.
- Original text by Project Gutenberg.
- Original text by Google Books