When a man has to urinate, but does not have the opportunity to do so,
one solution is to cover the tip of his penis with gum so urine does not
squirt out. When you continue to hold it eventually you will come to a point where you can no longer hold it. At this point, the gum fills with urine and becomes a water balloon. Usually used in context with holding off going to the bathroom for a long period of
time.
"Matt was in the office meeting so long he water ballooned" OR "OMG
George, where's the nearest restroom, I'
m water
ballooning over here!"