A way to get your siblings to leave you alone. It means that they arent really fat but you see them being that way. Or used as a really bad come-back when you absolutely have nothing else to say.
(Younger sister is bothering you)
Older sister: You are theoretically fat!
Younger sister: Haha thanks! (and leaves)
Person 1: Your ugly
Person 2: Your theoretically fat!
Person 1: Your mama's theoretically fat!
Person 2: No you didnt!