'Full Balloon' is a slang term used for a specific sexual act. It is when a man has anal intercourse with a partner (male or female), and when he is finished, he proceeds to urinate in the recieving person's anus, and the recieving person must hold it in. Thus the term 'Full Balloon'. Some believe the term originated in prisons, just as the term 'Salad Tossing' originated there as well.
"Dude, John gave Jack a Full Balloon! EEEWWW!"