mad as a hatter (or a March hare) completely crazy. informal
☞ In this expression, a hatter refers to Lewis Carroll's character, the Mad Hatter, in Alice's Adventures in Wonderland (1865). It is thought that hatters suffered from the effects of mercury poisoning because of the fumes arising from the use of mercurous nitrate in the manufacture of felt hats. The March hare version refers to the way hares leap about during the breeding season.