The word murder spelled backwards. It was first used in "The Shining" by Stephen King and then in both the film and the miniseries that were made based on the book.
Used in the episode 2.07 The Usual Suspects, when Dean is explaining the concept of anagrams during his 'Confession', Redrum is his example. More evidence that 'The Shining' is Dean's favorite movie.
See The Shining for more references to this movie in Supernatural.