Having to do with being shady, scandalous or otherwise devious. Appropriately used to describe someone with an ulterior motive of some kind, but can also be used to describe the action of a shady person(s).
Usually referring to something someone has done as "scheisty" (stealing from a friend, cheating on your girlfriend) goes hand in hand with saying that the person who did it is scheisty as well.
You guys have been boys since the fifth grade!
You're straight scheisty, playa.
That fat girl I took to prom ended up costing me well over five-hundred bucks on my tux alone...then cheated on me at the afterparty, but for some reason I still can't get enough of that scheisty flip.