A garboon is a more specific racist term than words such as "nigger" or "coon". It refers to large black men (or women?) with particularly stereotypically "black" features, the likes of which would be found in a racist portrayal, e.g. enormous lips, wide and flat nose, extreme prognathism. Possible etymologies include the word "baboon" and "Gabon" (an African country) and the fact that the word sounds like some species of ape.
Did you see that garboon with the horizontal vagina of a mouth on his face?