Old definition: madness induced by mercury fumes generated in factories that used this metal in hat fabrication.

Millenial definition: condition experienced by ambitious women who have to assume many roles in order to maintain diplomacy and harmony in a male dominated world.

The field of dentistry has confirmed the validity of this new definition by claiming that introduction of composite materials to replace amalgam restorations have shown no quantifiable impact on the behaviors of driven female practitioners.
My girlfriend has a mad hatter disease, Friday is always her second favorite F* word.
by Goldstar July 11, 2015
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If someone says you are mad as a hatter, they are accusing you of being quite irrational. The sense of madness here is "suffering from a disorder of the mind; insane." This phrase usually refers not to someone who is actually insane, but rather to a more normal person who is behaving in an irrational way.
The phrase emerged in England in the 19th century. Hatmakers in those days used a lot of felt that was treated with chemicals including lead, arsenic, and mercury.
Unfortunately, those chemicals are highly toxic. The symptoms of such poisoning include palsy, confused speech, and distorted thinking. Today, making hats is a much safer profession, but the phrase survives.
An interesting alternate explanation of the phrase derives hatter from Anglo-Saxon atter (poison), which is related to adder (a poisonous snake whose bite was thought to cause insanity).
by Its me, its me, the M-E-T-Z! December 9, 2003
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This goes back to the time when the people in the hat industry wound up with mercury poisoning due to the process of manufacturing the felt for hats.
He is as mad as a hatter,
by WDD2 April 28, 2016
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A fine cocktail made with equal parts of beer gin and absinthe
Dude! I got TRASHED yesterday with your swiss mad hatter
by TheGarzo January 29, 2022
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The act of cumming on your partner’s hair and immediately placing a hat on afterwards. Thus, making them a mad hatter from the amount of time it will take to completely remove your baby sauce.

Invented in Danbury Connecticut; home of the Mad Hatters
“Did you hear Conway gave Cathy a mad hatter. There was so much cum it glued the hat right onto her head”
by Mike Honcho playgirl star November 21, 2019
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A deeply annoying person who you just wish would leave you alone.
"Connie, Megan and Aimee are a bunch of mad hatters" Chef Adrian
by wolfburp November 15, 2016
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Mad Ass-Hatter is that one guy /girl who is absolutely batshit crazy and also an asshat/jackass.
Usually reserved for exes, enemies, or frenemies.

Can be used as an action word as well to describe weird situations/drama.
Example 1:

Person A: Dude, Jack is such a mad ass-hatter.
Person B: I know right? Can’t believe I dated that fucking psychopath.

Example 2:

Person A: Yo, did you just see what she did to George?
Person B: Yeah, that was some weird mad ass-hattery, my guy.
by BADDYBIDDY February 24, 2019
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