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?