From Longman Dictionary of Contemporary Englishhatterhat‧ter /ˈhætə $ -ər/ noun [countable] old-fashioned BODCCsomeone who makes or sells hats → as mad as a hatter at mad(2)
Examples from the Corpus
hatter• From what I can gather he was as mad as a hatter, and really no good at all.• Or he's mad as a hatter.• Also patron of fullers and hatters.• I had the family-planning abilities of a mad hatter.• So both the March hare and the mad hatter are very mad.• The mad hatter, the March hare and the dormouse.• Thelma Speirs is a professional hatter, and one half of the design duo Bernstock & Speirs. 