Top Definition
Protruding fat mass between the belly-button and the upper pelvis whose shape has been severely affected by the action of gravity.

Demographically, the belly underflap is most common in senior men who drink a lot of beer and women in their thirties who tend to choose extremely tight clothes.

Belly underflap is also observable in successful biggest loser contestants
boy 1 - "Hey dude! Check-out that girl's belly underflap!"
boy 2 - "Yeah! Yesterday she was totally hawt, but today she looks like T-1000 in the molten iron pool!"
by jtuf January 06, 2012

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