fraternize (v.) Look up fraternize at Dictionary.com
1610s, "to sympathize as brothers," from French fraterniser, from Medieval Latin 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."
A piece of frat, Wren-language for any attractive young woman -- ex-enemy -- in occupied territory. [John Irving, "Royal Navalese," 1946]
Related: Fraternized; fraternizing.