To associate with others in a brotherly or congenial way.
To associate on friendly terms with an enemy or opposing group, often in violation of discipline or orders.
[French fraterniser, from Medieval Latin frāternizāre, from Latin frāternus, fraternal; see fraternal.] frat'er·ni·za'tion (-nĭ-zā'shən) n., frat'er·niz'er n.