The phrase “mad as a hatter” has an interesting origin tied to real-life occupational hazards. It comes from the 18th and 19th centuries when hat-making was a common trade, and mercury nitrate was used in the production process. Hatters, or people who made hats, were frequently exposed to mercury fumes as they worked with felt, which was often treated with mercury. Prolonged exposure to mercury had severe neurological effects, causing symptoms like tremors, irritability, and hallucinations—a condition sometimes called “mad hatter syndrome.” As a result, people observed the peculiar behavior of some hatters, leading to the phrase “mad as a hatter” to describe someone acting erratically or irrationally.

The phrase gained additional popularity through Lewis Carroll’s Alice’s Adventures in Wonderland, where he introduced the “Mad Hatter” character, whose strange behavior seemed to match the image of an unhinged hatter, even though Carroll didn’t specifically reference mercury poisoning.