The literal meaning of "hostia" is the consecrated wafer used in religious rituals, but since religion is a very important part of the popular culture, it has acquired many meanings as a cursing in Spain with many meanings.
In this case, the whole expression refers to a place that is very far away from the speaker.
Other synonymous expressions:
- En el quinto infierno (Mexico)
- En el quinto palo dulce (I've heard this expression among South Americans, but I don't know how popular it is.)
There are a few others using "quinto", but they use even worse words.
Some examples:
- No quiero ir a tu casa, vives hasta el quinto infierno.
- Nos perdimos varias horas hasta que llegamos a una casa en la quinta hostia.
- Voy a ir en auto porque esta dirección queda hasta el quinto infierno y allá no llega el autobús.
For more meanings, take a look at the
DRAE.
By the way,
The Free Dictionary has good translations for a few idiomatic expressions with the word.
Ask if you need more examples.