(as) mad as a hatter

Also, mad as a March hare. Crazy, demented, as in She is throwing out all his clothes; she's mad as a hatter. This expression, dating from the early 1800s, alludes to exposure to the chemicals formerly used in making felt hats, which caused tremors and other nervous symptoms. The variant, dating from the 14th century, alludes to the crazy behavior of hares during rutting season, mistakenly thought to be only in March.

If someone is as mad as a hatter, they are crazy. Her sister's as mad as a hatter and if you ask me she's not much better herself. Note: In the 19th century, `hatters' or hat-makers used nitrate of mercury to treat their fabrics. This substance is poisonous, and if the hat-makers breathed it in, they often suffered brain damage. As a result, hatters were traditionally thought of as mad. In Lewis Carroll's children's story `Alice in Wonderland' (1865), one of the characters is a hatter who behaves very strangely. Carroll may have based the character on a well-known Oxford furniture dealer, Theophilus Carter, who was known as the `Mad Hatter'.

Crazy; mentally deranged.

Crazy. Although many readers associate mad hatter with Alice’s tea party in Wonderland, attended by both a March Hare and a Hatter, the term is considerably older and is thought to come from the fact that the chemicals used in making felt hats could produce the symptoms of Saint Vitus’ dance or other nervous tremors. In literature, references occur in Thomas Haliburton’s The Clockmaker (1837) and Thackeray’s Pendennis (1850), both predating Lewis Carroll’s Alice’s Adventures in Wonderland (1865). The synonym mad as a March hare, incidentally, which dates from Chaucer’s time, is virtually obsolete.

Crazy. The standard explanation comes from the effect to the brain caused by mercury nitrate used by 18th- and 19th-century hatmakers. Another view holds that “mad” originally meant “poisonous” and “hatter” is a corruption of the Saxon word “atter,” the adder snake, the bite of which affects the brain. In any event, the Mad Hatter character in Lewis Carroll's Alice in Wonderland is a testimony to eccentricity bordering on madness.