The act of securing the shaft of the penis, when erect, in the waistband of your trousers, boxers, sweat pants etc.. such that the head protrudes above the garment in the spot a belt buckle would be.
Person 1: Hey, I just got back from Australia, would you like to see the belt buckle I got there?
Person 2: Sure!
Person 1: *Exposes testicles*
Person 2: Oh god!
Person 1: That's my Australian Beltbuckle. It comes from down under.