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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The better hat, the most bad ass looking hat worn in the room. The baddest hat.
That's a swell hat Matt's wearing but Devin's is by far the badder hatter.
by Youmadbrubadtothebougie October 27, 2020
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A totally consumed Conspiracist that is CONSTANTLY trying to convince you that conspiracies are true.
I cant believe all the wild and crazy stories you keep telling me man, I don't want to hear it - your a Tin Hatter.
by Busters March 18, 2010
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Pejorative. A male homosexual, so-called due to the propensity of the tip of the male member to become coated with feces during the act of anal intercourse.
Boy George is the king of the brown hatters.
by Tunmy AuGratin February 25, 2006
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One of the new-found "fans" of the Boston Red Sux who has limited to no knowledge of the game of baseball, and who buys expensive and scarce tickets to the ball game because of the frat-party like atmosphere, and so he or she can brag of their allegiance around the water cooler. The pink hatter is so named because their team apparrel purchases are unconventional in color, and are merely chosen in an attemp to call attention to themselves and their amplified hipster factor. Pink hatters can be found in the ballpark taking the seat of a more interested and less casual fan that was unable to get a ticket.
Did you see the movie Fever Pitch where Drew Barrymore gets hit in the head by a baseball at a game because she is using her lap-top computer? That scene was meant to be humorous, but it made light of the serious problem of the confluence of pink hatters to Fenway Park. Geez, bandwagon fannery has certainly gotten out of control.
by Hipster Hater March 29, 2008
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A defense to trolling, most commonly on World of Warcraft forums, that involves saying that the troll "can't stop hiding behind alts and troll on their main." Usually improperly punctuated with poor grammar.
Near perfect execution of the hatter defense:

"Is it because I have what little balls it takes to post on my main, I mean I understand the point of some trolls its either they want some anonymity, which makes no sense seeing how your in a faceless game, or they want to make you mad because you don't know their main and can't bug them"
by KalametyTheRogue April 13, 2010
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Tinfoil Hatter is a normally derogatory nickname for a conspiracy theory buff, as the stereotypical image of one usually includes a tinfoil hat worn on the head to block out the mind control waves or chemtrails.
Did you hear that Ryan got coded at school for wearing a tinfoil hat around class?
Dang, I knew he was paranoid, but not that much of a tinfoil hatter!
by sebacostamr November 23, 2018
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