When someone (usually a very obese person) has a nipple with some extra baggage on the bottom of it.
"Dude did you see that guy?!?!?! His tits were hella saggy!!!" "Yeah, I think he has a Nipple Sack!!! Nipple Sack
The area around your weenus sometimes bigger than others, but is usually very fun to play with. Unknown to most doctors but is the place ingredients for breast milk is stored until needed. The Nipple Sack is very important for both males and females.
girl 1: "Can I touch your nipple sack?"
boy 1: "I'm a dude!"
girl 1: "Silly your nipple sack is the thing around your weenus!"