A gerdundermuffin is the act of climbing on top of a public toilet stall, doing the splits, and shitting into the toilet below. This is a feat, and when accomplished, the gerdundermuffiner should have a lot of pride.
Andy: Oh, God! Why is there shit splattered all over the toilet seat?!
Jacob: I'm sorry. I attempted a gerdundermuffin, but failed miserably.