A person who makes or sells hats.
From 'hat' (Old English 'hæt') combined with the agent suffix '-er' meaning one who does something. People who made felt hats sometimes experienced mercury poisoning, leading to the phrase 'mad as a hatter.'
The 'Mad Hatter' in Alice in Wonderland references a real problem—hat makers used mercury in felting, and mercury poisoning caused tremors, mood swings, and strange behavior. So 'mad as a hatter' originally meant someone whose strange behavior was caused by actual brain damage from their job!
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