“Mad as a hatter” is a colloquial phrase used to refer to a crazy person. In 18th and 19th century England mercury was used in the making of felt, which was used in the manufacturing of hats. People who worked in these hat factories were exposed to trace amounts of the metal, which accumulated within their bodies over time, causing some to develop dementia caused by mercury poisoning. Thus the phrase “Mad as a Hatter” became popular as a way to refer to someone who was perceived as insane.
The first documented instance of the phrase can be found in the 1829 short story, Noctes Ambrocianæ, published in Blackwood’s Edinburgh Magazine:
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.
SHEPHERD (to TICKLER.): Dementit.
ODOHERTY (to both.): Mad as a hatter. Hand me a segar.
The next known documented instance of it appears in the 1835 work The Clockmaker, by Canadian Thomas Chandler Haliburton:
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 later in the same work) 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.
The leading theory as to the origin of the phrase is that it refers to a genuine condition that began afflicting certain hat makers in the 17th century called “mad hatters’ syndrome” or “hatters’ shakes”. The symptoms associated with this condition were first described in full detail in 1829 by a Russian physician, the same year the first known instance of the expression came about.
It continued to be a problem for hat makers through the 19th century in much of the Western world, though in the U.S it was a phenomenon that continued into the mid-20th century for reasons we’ll get into in a bit.