informal Completely insane.
Popularized with reference to Lewis Carroll's character the Hatter in Alice's Adventures in Wonderland (1865), although the phrase was first recorded in the 1820s; the allusion is to the effects of mercury poisoning from the former use of mercurous nitrate in the manufacture of felt hats
- 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.
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