In Norse Mythology, Niflheim is one of the Nine Worlds.
In author H. Beam Piper's Terro-Human Future History , Niflheim was an inhospitable planet similar to the Judeo-Christian concept of Hell. Consequently, the planet is adopted as an expletive throughout much of the explored Galaxy.
Niflheim has approximately the same definition and emotional impact as the American English slang term, "hell.
In Uller Uprising[]
Uller Uprising by H. Beam Piper
In the introduction to Piper's novel Uller Uprising, the planet Niflheim is described in intimate detail. It contains an essay by Dr. John D. Clark describing in the planetary system of the star Beta Hydri, and gives the names of those planets: Uller and Niflheim.
A publisher's note stated that Clark's essay was written first, and given to the contributors as background material for a novel they would then write.
Uller Uprising -- as are all but one of Piper's works -- is now in the public domain. It is available in a variety of e-reader formats and audio books at Project Gutenberg.