This expression is used to mean that you are not cowed or intimidated by anyone. You are in a position to stand up for yourself, strike back and/or take revenge on someone who is challenging you.
I'm gonna let the whole world see ... You gonna be on the news this evening, buddy. I'm gonna get seven on my side, watch.
By Clareta
I Split My Sides
when something that is humorous or witty someone may say "i split my sides" refering to them laughing very hard
tim: whats 300 pounds, lives in the attic, only eats pizza and only drinks beer?
john: i dont know, what?
tim: you
barry: i split my sides