This describes a homosexual act between two men, where the receiving partner has a sweaty ass, to the point that lube actually wasn't necessary, but lube was applied anyways, making for an extra slick experience.
Jim had already lubed up Bruce's asshole, but his bad case of swamp-ass made for an extra slick swampy.
When an "original" and/or "seasoned" swamp donkey has reached a high level of experience, they graduate from intercourse with anyone for a hit to only sleeping with married men. Usually only their "friends" husbands making them a high class swampy
Did you hear about Dana? She got caught in bed with Jennifer's husband... again! Guess that means she's a high class swampy now