Definition of mad as a hatterinformal + old-fashioned
- He's a nice fellow, but mad as a hatter.
: arising from, indicative of, or marked by mental disorder
: completely unrestrained by reason and judgment : unable to think in a clear or sensible way
: incapable of being explained or accounted for
: a fit or mood of bad temper
: anger, fury
: very, extremely
: one that makes, sells, or cleans and repairs hats
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