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The motion, and sound of one's balls swaying back and forth, and slapping a male's thighs when he is running up or down stairs with loose fitting shorts on.

NOTE : Not to be confused with Oh-my-god-that's-sicktapsis, which is when Meat Curtains hit thigh.
"I had a serious fit of demotapsis this morning. It was so bad that it woke my mom up!"
by Bzur December 21, 2009
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