What does "something is rotten in the state of Denmark" mean?

This is one of the most common allusions to Shakespeare's play, Hamlet. It can most likely be found in some form within all genres of literature of all time periods after the play was written. The speaker? One of Hamlet's cronies, Marcellus. The reasoning behind the line: Marcellus sees Hamlet go away with a questionable, ghostly figure (in this case in the guise of Hamlet's dead father). The line: "Something is rotten in the state of Denmark." This particular line is often alluded to when there is something questionable about a person's character or actions. How can this relate to theme? Well, in the case of Hamlet, it relates to the question of the ghost's honesty. Is this ghost from heaven or hell? Marcellus obviously thinks it's from hell. The Roman Catholics of Denmark of the time would have believed in the possibility of a spirit being from Purgatory (the place souls had to go to be purified before entering heaven) or coming directly from hell as a demon. There is no doubt that Marcellus (as Hamlet's friend and confidant) is of the latter opinion.