The Collaborative International Dictionary of English v.0.48:

Fraternize \Fra"ter*nize\ (? or ?; 277), v. i. [imp. & p. p.
   Fraternized; p. pr. & vb. n.. Fraternizing.] [Cf. F.
   fraterniser.]
   To associate or hold fellowship as brothers, or as men of
   like occupation or character; to have brotherly feelings.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fraternize \Fra"ter*nize\, v. t.
   To bring into fellowship or brotherly sympathy.
   [1913 Webster]

         Correspondence for fraternizing the two nations.
                                                  --Burke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fraternize
    v 1: be on friendly terms with someone, as if with a brother,
         especially with an enemy [syn: fraternize, fraternise]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "fraternize":
   act the traitor, associate with, assort with, be acquainted with,
   be friends, be inseparable, chum, chum together, chum with, clique,
   clique with, club together, collaborate, consort with, cotton to,
   fall in with, fellowship, flock together, fraternize with,
   hang around with, hang out with, herd together, hit it off,
   hobnob with, join, join in fellowship, keep company with,
   keep up with, know, mingle with, mix with, pal, pal up with,
   pal with, quisle, run in couples, run with, sort with,
   take up with, tie up with, turn against