All Personel, South East Asia Theater of War: Upon receipt and thereafter, you will cease refering to our allies as slopes, gooks, slants and yellow bastards and afford them the respect due to the worthy oriental gentlemen that they are.
Viscount Robert Mountbatten Commanding General South East Asia
While no etymologist I, it would seem from your research that Mountbatten was engaging in some particularly Churchillian word play.
Surprising, though, was your unfamiliarity with the term. Robert Ruark built two African novels on it; Uhuru and Something of Value. If you have not read either or both of these you, in particular, should not eat nor sleep until you have copies. It will be worth your while.
Why was that surprising?