Usually used by freshman in highschool to annoy the upper class men. The actual meaning of soump is, "Look who's talking" and is generally a comeback to an insult.
Jane: Well, Donny and I went out and he revealed his trump stump. I think he could tell I was disappointed in the tiny nub because the next thing that happened was it was just a small flaccid blob of dough. We laughed and called it trump slump.