mad as a hatter

Disputed. Possibly from Old English ātor (“poison”), and thus related to English atter. Alternatively from hat-makers suffering from Korsakoff's syndrome due to handling mercury-contaminated felt.

• (Received Pronunciation) IPA^(key): /ˌmæd æz ə ˈhætə/
• (US) IPA^(key): /ˌmæd æz ə ˈhætɚ/

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mad as a hatter (not comparable)

1. Demented or crazy.
   • 1857, Thomas Hughes, Tom Brown's Schooldays, Part II, chapter 3,

     He's a very good fellow, but as mad as a hatter. He's called Madman, you know. And never was such a fellow for getting all sorts of rum things about him. He tamed two snakes last half, and used to carry them about in his pocket; and I'll be bound he's got some hedgehogs and rats in his cupboard now, and no one knows what besides.

   • 1895, John Kendrick Bangs, A House-Boat on the Styx, chapter 7,

     "I think he’d be as mad as a hatter at your insinuation that he would invite any of his wives, if all I hear of him is true; and what I’ve heard, Wolsey has told me."

   • 1904, G. K. Chesterton, The Napoleon of Notting Hill, Book III, chapter 3,

     I think Adam Wayne, who is as mad as a hatter, worth more than a million of you. But you have the force, and, I admit, the common sense, and he is lost.

   • 1920, Agatha Christie, The Mysterious Affair at Styles, chapter 10,

     Sometimes, I feel sure he is as mad as a hatter; and then, just as he is at his maddest, I find there is method in his madness.

   • 1939, Agatha Christie, And Then There Were None, chapter 11,

     If you ask me that woman's as mad as a hatter.

• disturbed, insane, loopy, nutty
• mad as a March hare
• See also Thesaurus:insane

• hatter
• Mad Hatter