You can read the chapter where the word was met. And I put a little extract below:
To me the convalescent would it now be suffering and torment to believe in such phantoms: suffering would it now be to me, and humiliation. Thus I speak to backworldsmen.
Suffering was it, and impotence – that created all backworlds; and the short madness of happiness, which only the greatest sufferer experienceth.
Weariness, which seeketh to get the ultimate one leap, with a death-leap; a poor ignorant weariness, unwilling even to will any longer: that created all gods and backworlds.
There is also a derived word "backworldsmen". I guess the world is author-invented? Could you shed some light on its meaning?
Best Answer
Google does give results for Thus spake Zarathustra backworld. The first one is http://nathanbauman.com/odysseus/?p=2949
The first paragraph of that page says (my emphases):
This isn't a German-language site so attempting to translate the German Hinterwelt and Hinterweltlern is not going to be easy. "Back-world" and "back-worlds-men" are the literal translations.
But the expression other-worldly does exist in English: "belonging or relating to, or resembling, a world supposedly inhabited after death" [Chambers], or perhaps "a world outside the physical realm."