It is just like a threesome only it goes on forever.
guy 1: OH man! Last night my gf and her bestfriend had this threesome and god it just went on and on!!
guy 2: You mean a pi-some?
guy 1: A what?
guy 2: You know its like a threesome only it goes on for what seems like forever?
guy 1: I guess you can call it that.