The act of straddling over another's body that is sitting on the toilet while both people defecate into the toilet and gaze into each-others eyes. The act is supposed to look like spider legs hanging from a toilet thus giving it the name spiderpoop.
Guy: Hey, lets do something neat.
Other Guy: I know, lets spiderpoop.
Guy: That sounds groovy man.
Dude: I just spiderpooped your Mom last night.