To fill a cave is to have a big steamy shit, about the size of a bag of sugar! The name comes from the time two marine explorers in greece came among a cave whilst swimming. The decided to try and venture in only to find a huge steamy shit in the cave.
person 1: Your boppin like a break dancer whats wrong man?
person 2: Aw man i need to fill a cave desperatly and there is no toilet