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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.
by roshmacack August 08, 2012