The Collaborative International Dictionary of English v.0.48:

Mob \Mob\, n. [L. mobile vulgus, the movable common people. See
   Mobile, n.]
   1. The lower classes of a community; the populace, or the
      lowest part of it.
      [1913 Webster]

            A cluster of mob were making themselves merry with
            their betters.                        --Addison.
      [1913 Webster]

   2. Hence: A throng; a rabble; esp., an unlawful or riotous
      assembly; a disorderly crowd.
      [1913 Webster]

            The mob of gentlemen who wrote with ease. --Pope.
      [1913 Webster]

            Had every Athenian citizen been a Socrates, every
            Athenian assembly would still have been a mob.
                                                  --Madison.
      [1913 Webster]

            Confused by brainless mobs.           --Tennyson.
      [1913 Webster]

   3. A criminal organization or organized criminal gangs,
      collectively; the Mafia; the syndicate; as, he was a
      lawyer for the mob.
      [PJC]

   Mob law, law administered by the mob; lynch law.

   Swell mob, well dressed thieves and swindlers, regarded
      collectively. [Slang] --Dickens.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Swell \Swell\, a.
   Having the characteristics of a person of rank and
   importance; showy; dandified; distinguished; as, a swell
   person; a swell neighborhood. [Slang]
   [1913 Webster]

   Swell mob. See under Mob. [Slang]
      [1913 Webster]