The phrase "mad as hatter" is often used to describe someone who is completely insane or irrational. It originates from the fact that hatters, who make hats out of animal fur, were once exposed to harmful chemicals like mercury, which led to mercury poisoning and resulted in erratic behavior. If we were to look for antonyms for "mad as hatter," we could think of terms like "sane," "rational," "calm," and "level-headed." These words portray individuals who are in control of their emotions, thoughts, and actions. They can also be described as composed, sensible, and reasonable, as opposed to being wild, eccentric, and irrational.