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When two people push their assholes together and alternately shit into the others asshole, hence boomerang.

Invented by Bill Cosby in 1956.

See also ultimate dirtpipe boomerang showdown
Kevin:"What are you watching?"
Bill:"Two black trannys doing a dirtpipe boomerang."
Kevin:"Pass the KY jelly"
by TheChode April 02, 2007