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@mrs-trebus16 wrote:

In olden times, hatters held mercury in their mouth to spray on to top hats and was buffed to a shine. Due to the mercury affecting the memory and causing loss of senility, this is how the term 'Mad as a hatter' came about

 

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 Another interpretation is that 'Mad as a hatter' came from 'Mad as an adder', i.e. 'As angry as a snake'.  It got distorted over time. 🙂