But what exactly is
Dictionary.com actually has an entry for the term, stating:
Swamp ass involves a sweaty wetness in the butt crack or general nether region that may soak through the underwear, even the pants. Chafing and funky odors are also common symptoms. Beware.
Example of an unfortunate victim of Swamp Ass:
But where did the term come from? Dictionary.com says that the words have been used since at least 1995 when an anonymous internet user said to another user “your mother smells like swamp ass” during an argument about hockey.
“Swamp Ass” made its Urban Dictionary debut in 2002: