We found 19 dictionaries that define the word mad as a hatter:

General (13 matching dictionaries)

1. mad as a hatter: Merriam-Webster
2. mad as a hatter: Oxford Learner's Dictionaries
3. mad as a hatter: Collins English Dictionary
4. mad as a hatter: Wiktionary
5. mad as a hatter: Dictionary.com
6. Mad as a hatter: Wikipedia, the Free Encyclopedia
7. Mad as a Hatter: Dictionary of Phrase and Fable (1898)
8. Mad As A Hatter: A to Z list of the most popular idioms
9. Hatter, Mad as a: The Word Detective
10. mad as a hatter: TheFreeDictionary.com
11. Mad as a hatter: World Wide Words

Business (2 matching dictionaries)

12. mad as a hatter: Legal dictionary
13. mad as a hatter: Financial dictionary

Computing (1 matching dictionary)

14. mad as a hatter: Encyclopedia

Medicine (1 matching dictionary)

15. mad as a hatter: Medical dictionary

Miscellaneous (1 matching dictionary)

16. mad as a hatter: Idioms

Slang (1 matching dictionary)

17. Mad As a Hatter: Urban Dictionary

Definitions from Wiktionary (mad as a hatter) ▸ adjective:  Crazy or demented. Similar: mad as a March hare, mad as a mongoose, mad as a fish, lunatic, looseheaded, half-cracked, demented, off-minded, mad as a cut snake, crazy, more... Opposite: sane, rational, lucid, clearheaded Phrases: as mad as a hatter, more... ▸ Words similar to mad as a hatter ▸ Usage examples for mad as a hatter ▸ Idioms related to mad as a hatter ▸ Wikipedia articles (New!) ▸ Popular nouns described by mad as a hatter ▸ Words that often appear near mad as a hatter ▸ Rhymes of mad as a hatter ▸ Invented words related to mad as a hatter