The world, society, or realm of criminals and criminal activity taken as a whole.
From criminal + -dom (Old English suffix meaning 'state, condition, realm'), similar to kingdom or Christendom. Emerged in 19th-century literature to describe the underworld as a distinct society.
Victorian and Edwardian writers loved this word because it made the criminal underworld sound like an actual kingdom with its own rules and culture—think of it as the 'opposite country' within a country.
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