The Collaborative International Dictionary of English v.0.48:

Jacobin \Jac"o*bin\ (j[a^]k"[-o]*b[i^]n), n. [F. See 2d Jack,
   Jacobite.]
   [1913 Webster]
   1. (Eccl. Hist.) A Dominican friar; -- so named because,
      before the French Revolution, that order had a convent in
      the Rue St. Jacques, Paris.
      [1913 Webster]

   2. One of a society of violent agitators in France, during
      the revolution of 1789, who held secret meetings in the
      Jacobin convent in the Rue St. Jacques, Paris, and
      concerted measures to control the proceedings of the
      National Assembly. Hence: A plotter against an existing
      government; a turbulent demagogue.
      [1913 Webster]

   3. (Zool.) A fancy pigeon, in which the feathers of the neck
      form a hood, -- whence the name. The wings and tail are
      long, and the beak moderately short.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Jacobin \Jac"o*bin\, a.
   Same as Jacobinic.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Blackfriar \Black"fri`ar\, Black friar \Black"
fri`ar\(bl[a^]k"fr[imac]`[~e]r). (Eccl.)
   A friar of the Dominican order, so named because wearing the
   black mantle of the Dominicans; -- called also predicant
   and preaching friar; in France, Jacobin. Also, sometimes,
   a Benedictine.

   Syn: Dominican.
        [1913 Webster]

WordNet (r) 3.0 (2006):

Jacobin
    n 1: a member of the radical movement that instituted the Reign
         of Terror during the French Revolution

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "Jacobin":
   Bolshevik, Bolshevist, Bolshie, Carbonarist, Carbonaro, Castroist,
   Castroite, Charley, Communist, Cong, Fenian, Guevarist, Leninist,
   Maoist, Marxist, Mau-Mau, Puritan, Red, Red Republican, Roundhead,
   Sinn Feiner, Trotskyist, Trotskyite, VC, Vietcong, Wobbly, Yankee,
   Yankee Doodle, anarch, anarchist, anarcho-syndicalist,
   bonnet rouge, criminal syndicalist, extreme left-winger, extremist,
   left-wing extremist, lunatic fringe, mild radical, nihilist,
   parlor Bolshevik, parlor pink, pink, pinko, radical, rebel, red,
   revolutionary, revolutionary junta, revolutioner, revolutionist,
   revolutionizer, sans-culotte, sans-culottist, subversive,
   syndicalist, terrorist, ultra, ultraist, yippie