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.
To associate closely with inhabitants of an enemy country, esp to consort sexually with the women (WWII armed forces)