The Collaborative International Dictionary of English v.0.48:

Nixie \Nix"ie\, n.
   1. See Nix.
      [1913 Webster]

   2. Nothing. [Slang]
      [Webster 1913 Suppl.]

   3. (U. S. Mail Service) A piece of mail matter which cannot
      be delivered, either because no post office exists at the
      place to which is it addressed, or because there is no
      place of the name mentioned in the designated State,
      Territory, or the like. [Cant]
      [Webster 1913 Suppl.]