To use the toilet.
It originated from the idea of the old-fashioned above-ground outhouses that stunk up a mile radius. Signs on the doors would instruct the users to scoop some sawdust, provided in the stall, into the tank to prevent odors from permeating.
Can also be phrased "add a scoop"
Dude, where you going? We're about to jump in the lake!
Just a second guys, I'm gonna add a scoop of wood.
