A Duoble Gulp is an extreamly big soda, fountain drink you can buy as 7 11. It is quite a deal, a new cup is about $1.70 and a refill is $1.00. It also make for a great dildo.
When you stick a double gulp in your bunghole bottom first and leave the straw hanging out. Then the woman/man enjoys a tasty soda treat out of said straw.