It means 'extremely crazy'.
Milliners (hat-makers) used to use Mercury nitrate to soften the felt that went into making the hats. Breathing mercury vapors leads to a life-threatening condition known as mercury poisoning; the damage to the brain leads to slurred speech, loss of coordination, dementia, hallucinations, and finally death.
This stereotype of the mad hatter is famously personified in Lewis Carroll's - and later, Disney's - famous character of the same name. These days, anyone who has wild mood swings and a short attention span (especially if they're also clumsy) is at risk of being labeled 'mad as a hatter'.
The Mad Hatter is a hatter. He makes and sells hats for a living.
Mad as a Hatter was created in 1992.
No. In the original book, the mad hatter doesn't have a name, he isn't even called 'the Mad Hatter', he is simply called 'the Hatter'.
The March Hare is the Mad Hatter's friend.
Only the Mad Hatter is a hatter. The name "The Mad Hatters Tea Party" might suggest that there is more than one hatter there, but if you put the apostrophe in the correct place it becomes "The Mad Hatter's Tea Party", which means, "the tea party belonging to the Mad Hatter".
No, the Mad Hatter is a human being.
The Mad Hatter Mystery was created in 1933.
The Mad Hatter - album - was created in 1978.
Mad Hatter - album - was created in 1992.
Mad Hatter - comics - was created in 1948.
She got mad.
Mad Hatter was definitely Batman's villain