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An alien lifeform, or one who studies the extraterrestrial beings. Sean Oniell can be considered to be an expert because of his first hand experience with UFO's. Don't be confused with the common sex move the SHAWN ONEIL in which two gay lovers put sand in each other's buttcracks and try to scoop it out.
Man, he looks like a sean oniell, because of his huge head!
by MIckJaGGER November 10, 2006
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