Crazy, demented. The expression alludes to exposure to chemicals formerly used in making felt hats, which caused nervous symptoms. The variant 'mad as a March hare' alludes to the erratic behavior of hares during rutting season.
Examples:
- She is throwing out all his clothes; she's mad as a hatter.
Origin: Early 1800s; variant from 14th century
Synonyms: mad as a march hare