Mean Person:  You're fat.
Me:  Well, when the nuclear winter comes, I'm going to hide in my bunker and live off of my body fat.
Me:  And laugh while you starve to death.

Mean Person:  You have no friends.
Me:  That means that when I have achieved world domination, I won't have to share the power with anyone.
Me:  And that means that they won't be able to stop me from exiling you to a remote desert island in the middle of the Pacific.

Mean Person:  You're stupid.
Me:  When the zombie apocalypse arrives, I'll hide in my bunker and laugh because zombies only eat brains.  Which means that they'll ignore me.
Me:  So they'll eat your brains. 
Me:  Since you're obviously highly intelligent.