A Sub-nigger can be many different things, and the means of the word is mostly depending on location. Here's the most usual forms of the word.
1. A little boy or girl with black skin color.
2. A black person that loves hanging around underground ("sub" here refers to "subway")
3. Used by white racists who have particular grudge against asian, but also don't like black people meaning that an asian is something less than a nigger.
4. Used by black people on other black people that doesn't quite belong. They can be gay, short or something like that. A typical use of the word is rich niggers using it at poor niggers.
Doctor: Congratulations mam, it's a boy
Woman: Oh ma gosh, it's mah own little sub-nigger!
Person 1: Hey, have you seen Derrel lately?
Person 2: Nope, last time I saw him he were sleeping under a bench in a subway station.
Person 1: He's such a sub-nigger...
Asian hater: You're not even black, you're a yellow, fucking sub-nigger!
Afro american beggar: Can I please have some money for food?
Richie Rich (black): Get your own money, sub-nigger!