It ain't that kind of party.

Used to tell someone you're not going to do something.

Person A: "Hey, I'm going out to eat. Wanna come?"
Person B: "Sure, why not?"
Person A: "Alright."
Person B: "Um, could you buy my food?"
Person A: "...it ain't that kind of party."

by Panzermanathod August 1, 2007