Answer:
Mad as a hatter" is a colloquial English phrase used in conversation to suggest (lightheartedly) that a person is suffering from insanity. It is believed to emulate from Denton, Tameside, where men in the area worked predominantly in the hattery business, which used mercury in the hat making process. The accumulation of mercury in the body causes symptoms similar to madness. The earliest known appearance of the phrase in print is in an 1829 issue of Blackwood's Edinburgh Magazine.
