When one's stomach, or 'tummy,' sticks out further than one's breasts. Or in the case of men, pectorals. It is a very unattractive feature to say the least.
Iyun: Man, check out that guy Scotticus' Tummy Do
Hare Bear: Jeez, that shit is un-fucking-attractive. That dude should do P90X.
When a person's booty stick out more than their tummy do. Usually used for females:
Ex. Damn Leo, check out that dime with tummy do.