1610s, "to sympathize as brothers," from Fr. fraterniser, from M.L. fraternizare, from fraternus "brotherly" (see fraternity
). Military sense of "cultivate friendship with enemy troops" is from 1897 (used in World War I with reference to the Christmas Truce). Used oddly
by World War II armed forces to mean "have sex with women from enemy countries." Related: Fraternizing.