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The act of two guys engaging in a masturbatory battle, inside a dome-shaped jungle gym, where the winner is the combatant who can climax first. (Note: During the battle, the jungle gym should be covered by the wagering spectators to prevent any random onlookers from seeing inside.)

Term references the film Mad Max Beyond Thunderdome's arena where opponents battle each other to the death.
"Make sure there's no kids around! Bob and Bill are going beyond thunderbone! Two men enter, one man leaves!"
by Baron Marx May 21, 2013
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