<P> The origin of the saying may derive from: </P> <Ul> <Li> 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 . In 18th and 19th century England mercury was used in the production of felt, which was used in the manufacturing of hats common of the time . A late 19th - century example of the effect occurred with hatters in Danbury, Connecticut who developed a condition known locally as the Danbury Shakes . The condition was characterized by slurred speech, tremors, stumbling, and, in extreme cases, hallucinations . </Li> <Li> 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 ." </Li> <Li> 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 . Although this was presaged by political and religious radicalism, and was followed by a long married life . </Li> <Li> 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' ." </Li> </Ul> <Li> 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 . In 18th and 19th century England mercury was used in the production of felt, which was used in the manufacturing of hats common of the time . A late 19th - century example of the effect occurred with hatters in Danbury, Connecticut who developed a condition known locally as the Danbury Shakes . The condition was characterized by slurred speech, tremors, stumbling, and, in extreme cases, hallucinations . </Li> <Li> 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 ." </Li>

Where does the phrase mad hatter come from