Bully: Agh, you're ugly!
Me: Said your mom when you were born.
Bully: Agh, you're ugly!
Me: Said your mom when you were born.
Okay, so the bully says the other person is ugly. Then, BAM! The person comes back with "your mom said that when you were born!" which is like saying the bully is so ugly, even his own mom noticed when he came out. It's a classic roast, but you probably didn't get it because, let's be real, your brain runs on dial-up, not fiber optics.