As a Dead World
Limbo is a special dead world where you go to after you die in the Underworld. Only immortals are able to go to Limbo but as a result they can never leave Limbo and are trapped there for eternity.
In the theology of the Catholic Church, Limbo (Latin limbus, edge or boundary, referring to the "edge" of Hell) is a speculative idea about the afterlife condition of those who die in Original Sin without being assigned to the Hell of the Damned. Limbo is not an official doctrine of the Catholic Church or any other Christian denomination. Medieval theologians, in western Europe, described the underworld ("hell", "hades", "infernum") as divided into four distinct parts: Hell of the Damned (which some call Gehenna), Purgatory, Limbo of the Fathers or Patriarchs, and Limbo of the Infants.
Another word for Limbo could be the same as Mu. Mu is a Japanese and Korean word which denotes a negative: the absence of something. In Zen Buddhism, "Mu" is the answer given to an improperly phrased question that does not merit an answer. The Japanese reads more like "after death, the place they go; it's Mu". And in Zen Buddhism, mu is an answer to a question that depends on invalid axioms.
So when the rule states "the place they go; it's Mu", it means the question relies on an invalid assumption. They don't go anywhere, because there is nowhere for them to go to. This is why it gets anglicized as "Nothingness", because all that exposition is way too much detail for a notebook rule.
- In the Grim Adventures of Billy and Mandy Limbo is mentioned in the first episode,Grim chalenging Billy and Mandy at a limbo contest for the soul of Billy's hamster.