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A term commonly used when describing CT Side Saddle.
It is an optical illusion when you insert your thumb knuckle deep inside a girl's ass. Your knuckle will seem to disappear!
?: "Where'd it go?"
CT: "There it is!"
?: "Where'd it go?"
CT: "There it is! The disappearing knuckle!"
by Speed Racer 19 April 24, 2010