mad as a hatter

idiom

Definition of mad as a hatter

informal + old-fashioned
: severely mentally ill : crazy He's a nice fellow, but mad as a hatter.

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“Mad as a hatter.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/mad%20as%20a%20hatter. Accessed 9 Jul. 2020.

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