Variation on "nigger", evoking stereotypical African tribal chanting, dancing etc. Although very offensive today, many people wouldn't have regarded it as such in the past when it could also just mean "idiot".
"Straight outta the banana tree,eh? Well, let me tell ya this, nignog, you go stealing white man's motorcars, you give white man stick. Right?" - Film: Scum, 1977
"Sort yourself out, you nignog."