“Mad as a Hatter.”

Lewis Carroll may have popularized this phrase, but it was already widely in use before the mad hatter made his appearance in the Carroll’s classic Alice in Wonderland!

Many explanations ave been put forward for the origin of this cliché. “Mad as an adder” meaning “as dangerous as a poisonous snake” is one of them.

Hatters were believed to be driven to madness by the poisoning effect of mercurous oxide used in tanning felt.