Mad as a hatter means absolutely crazy. The most famous illustration of the phrase mad as a hatter occurs in Lewis Carroll’s Alice’s Adventures in Wonderland, with the character the Mad Hatter. In the nineteenth century, milliners or hatmakers used mercury in the processing of hats, and many succumbed to mercury poisoning. Mercury poisoning may cause mood swings, aggressiveness and other unpleasant social behaviors. The first printed occurence of mad as a hatter is 1835, though many believe that mad as a hatter derived from the phrase mad as an adder, meaning a snake. As of yet, there are no citations of mad as an adder predating the first known citation of mad as a hatter in 1835.
Chrissie, from Scotstounhill, Glasgow, had always wanted to take a ride in a hot air balloon and on Tuesday, bolstered by a dram of whisky and declaring “I’m as mad as a hatter”, she finally got her wish as a belated 100th birthday present. (The Courier)
In fact, in his book he confesses that “the naming of cats is a difficult matter … you may think at first I’m as mad as a hatter”, and there followed a flood of cat names to dumb the senses. (The New Zealand Herald)
“Paolo’s comments should be taken with a pinch of salt because he’s as mad as a hatter.” (The Evening Standard)
Max Rockatansky (Hardy), still mad as a hatter and haunted by torturous visions of his past, is kidnapped by a tyrannical warlord known as Immortan Joe (played by an unrecognizable Hugh Keays-Byrne, who also played the villainous Toecutter in Mad Max). (The Miami Herald)