mad as a hatter


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mad as a hatter

- Refers to the fact that hat makers suffered mental illness in the old days when they got mercury poisoning from treating fur.
See also related terms for mad.

mad as a hatter

Completely insane; from the former use of mercury in processing furs or hides to make hats. Hatters absorbed large amounts of this over the years which gradually poisoned them, often leading to mental disturbance.
References in classic literature ?
He must be as mad as a hatter," exclaimed the Colonel.