A slang "name call"; A reference to the character Cliff Claven from the show Cheers.
A master and avid sharer of trivia. Someone who never hesitates to set people straight with facts (or "facts" of dubious correctness), at every possible opportunity, whether or not the information is desired. Usually the stories are far longer than anyone wants to hear.
Person1: "It was just snow. Snow is snow."
Person2: "Well *actually* there, Normy, the Inuit have 37 different words to describe snow. Legend has it that the explorer, Pedro Ralfkin once became trapped in a snow bank, and..."
Person1: "You are such a Claven!"