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]