To be under the influence of hallucinagenic mushrooms. Can be used retrospectively or to describe someone's current condition.
I got so shroomed last night, that I thought little goblins were climbing through my window and had to hide in the closet for 3 hours....
The act of shitting all over a room and the most disgusting fashion hence: shit + room = shroom
"I had the worst case of the runs after I ate that burrito and shroomed all over the walls and mirror in the bathroom!"