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.
To sniff a recently used seat. This practice specifically refers to bicycle seat sniffing, though can refer to any seat. It is also usual practice to gaboon a seat that has been sat on by another, preferably unknown to yourself, person.