Swamp Tuba
Giving a reach-around handjob to an overweight man while spitting (including flem or small amount of vomit) into his anus, before tonguing it. Similar to a Rusty Trombone.
The person had a bad cold before giving a rusty trombone to a hairy fat dude, and then sneezed in his ass. It turned into a Swamp Tuba (or Swampy Tuba).