informal Completely insane: he’s indisputably a genius, but he’s also mad as a hatter
[with reference to Lewis Carroll's character the Mad Hatter in Alice's Adventures in Wonderland (1865), the allusion being to the effects of mercury poisoning from the use of mercurous nitrate in the manufacture of felt hats]
More example sentences
- She's mad as a hatter but that bunch of loonies will love her.
- ‘He is as mad as a hatter and in to everything,’ said friend Lesley Gill, who used to work with Brian in the Newbridge branch of Dunnes Stores.
- As long as you temper your unrestrained approach to life with occasional periods of sanity - and do your best not to get arrested - it's completely acceptable to be as mad as a hatter.
Definition of as mad as a hatter in:
- The US English dictionary