mad as a hatter

The phrase "mad as a hatter" typically refers to someone who exhibits eccentric or irrational behavior. It originated from the toxic effects of mercury used in hat-making during the 18th and 19th centuries. Chronic mercury exposure in hatters, who were constantly exposed to mercury vapor from the hat-making process, led to neurological symptoms like tremors, irritability, and memory loss. As a result, the phrase became associated with madness or insanity.