It's something you say after your male classmates with brains filled with illusions by the fact of your boobs are growing invite you to throw rocks at colorado local cars and SUVs.
Stan: Ah, hey, Bebe.
Stan: Uh, we're gonna throw rocks at cars later on, and we thought maybe you'd like to join us.
Bebe: Really? I've never done that before.
Butters: Wuuhh, it's really fun.You toss these little rocks at cars, and if the driver gets angry, you blame me.
Kyle: Yeah, it's cool.
Bebe: Why sure, that sounds hellafun.
The boys: Hahahahahaha...
Cartman: Right, heh.
Clyde: Wow, hellafun. That's awesome.
Token: I'm gonna start saying that now.