A person, generally one which the other has hooked up with, with whom contact of any sort is unwanted, most likely due to the overall awkwardness or displeasure that and sort of contact would bring. A situation of a sewage encounter is to be avoided by any means necessary.
Bro: Heads up serious Sewage heading our way man... What are we going to do?
Other Bro: Aww man I hate sewage. It's that girl that I hooked up with when I was totally blackout last night, and she is right between us and our future destination!