Not a Piece

Okay, so this joke is all about playing on words and throwing shade. The bully tries to insult someone by calling them a "piece of shit." But the person being bullied is too smart. They hit back by saying they aren't a "piece" (like a literal object) and they aren't "brown" (implying a racial undertone to the insult). It's a savage comeback, especially for someone as dumb as you.