Under flap 

The crusty film that forms made of sweat that collects under a breast, usually a large breasted girl, or woman.. or anyone who has breasts large or saggy enough to cause and form pockets, if its large enough to hide items, its large enough to collect dirt sweat lint and small creatures. Liken it to swamp ass or belly button lint..

Dude, she flashed us and under flap fell out.

Much to his surprise and horror when he got done licking there, there was some under flap stuck in his teeth

Under flap by SLAA addict July 12, 2012

Underflap 

back side of the ball sack

"Bro, my underflap is so itchy!"

Underflap by koolaidjollyman September 28, 2009

belly underflap 

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!"

belly underflap by jtuf January 6, 2012

underlap 

(proposed definition)

Two areas underlap when there is a gap between them - rather than just touching.

When hanging wallpaper, you should arrange the sheets so that they neither overlap nor underlap.

underlap by Ed Avis June 21, 2003

underflamer 

One who is in the closet but is too timid to do much more than open the door on occasion and peek out.

A good underflamer is hard to find. I wonder why?

underflamer by Michael Ely January 27, 2004

underlap 

The opposite of wordoverlap/word, which technically means to skip or miss a part of something.

My fucking retarded teacher underlapped the design project, so we couldn't get it all.

underlap by EC April 23, 2003

underlap 

to support from below; having strengths that work together without conflict

His personality underlapped hers and strengthened their team.

underlap by Dennis P Ryan January 24, 2009