1610s, "to sympathize as brothers," from French fraterniser, from Medieval Latin fraternizare, from Latin 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 in World War II armed forces jargon to mean "have sex with women from enemy countries" as a violation of military discipline.
A piece of frat, Wren-language for any attractive young woman -- ex-enemy -- in occupied territory. [John Irving, "Royal Navalese," 1946]
Related: Fraternized; fraternizing.