As mad as a hatter

Origin

The phrase emerged from the grim reality of 19th-century hat making, particularly in England. Hatters frequently used mercury nitrate in the felting process for beaver and rabbit fur, which produced stiff, fine hats. However, prolonged exposure to mercury vapor led to severe neurological damage, causing tremors, slurred speech, memory loss, and extreme irritability—symptoms collectively known as "mad hatter disease" or erethism. Witnessing hatters exhibiting these bizarre and erratic behaviors made the comparison to their supposed madness a common and chilling observation, solidifying the phrase in the popular imagination long before Lewis Carroll’s character.