The act of several friends/co-workers/people taking turns shitting in a toilet without flushing in between. The shit is then left for an unsuspecting victim to walk into (this is perfect for bosses or your friends' girlfriends). The ideal dumpstack includes throwing the used toilet paper in the trash instead of the toilet so as not to cover the awesomeness of the stacked shit. Must include at least three dumps to constitute a stack.
"Man when Carl comes in tomorrow he's gonna find the most wicked dumpstack of all time. There's at least five dumps in that toilet right now."
"I hate Billy's girlfriend. I say we give her toilet a dumpstack at her party Friday."
A dump stack is achieved by eating a meal that pushes out a lingering dump in your innards. The pressure on the existing dump caused by the ingestion of the meal gives the push needed to clear things out.
I ate that sandwich, dump stacked, and blasted a Hot Carl all over her.