Ah, yes. The incest insult. He's heard it slung so many times it's become something that Trevor now actively enforces the idea of, because, you know, why not.

"You know it, asshole! Daddy and his sister had a baby and it's gonna drag you right down to hell."

He yanks the car into drive, seeing the kid's picking his stupid bike back up. Is he going to run? Is Trevor going to chase down and terrify some hapless high-schooler? We just don't know. (But most likely.)