OOK in the middle.
From Korea, then ‘nam. Might go back to WW II.
Seems the wars develop a lot of terms for their adversaries. It's not always racial either. WW2 gave us Kraut, WOP, Heinie, Fritz, Hans, Jerry, nip, Japs, and yes the G00k word, russkie, ivan.
The military has a rich history in naming conventions for those they fought.