Is it when you take a dump and haven't whiped good enough and it starts to get sweaty and itch? 
Yeah or if your o-ring is shot and its 100 and muggy. 
For me, it's when you are sitting for a really long time and your ass starts to sweat, and your shorts or pants or underwear or all them together start to form a weird bond with your ass that can only be separated by an awkward stand and pick.
it's when it's really hot and your ass crack sweat mixes with left over shit stain and the resulting mess gets all over your ass
It's also what I named every disease on that Plague Inc game. "Swamp Ass had infected 100% of Australia." You're goddamn right it has
It's when the cavern between your 2 thighs and your ass that is formed while sitting down develops its own ecosystem, complete with weather patterns and plant life.
Florida is the swamp ass capitol of the United States. Its when you have to antique your balls and underwear with Gold Bond powder because its so fucking humid you cant walk 3 feet without getting all sweaty and... swamp assy
Lebron applying Gold Bond to someone in the upper deck to help prevent Swamp Ass during the game.
