In the word "arbeit-s-los", a linking "s" is used to combine both parts of the word.
As copied from the articel:
The linking "s" is used when the first word of the compounding ends with one of the following syllables: -tum, -ling, -ion, -tät, -heit, -keit, -schaft, -sicht, and -ung.
For the case of "Arbeit", it is handled like "-heit" or "-keit", although it is not in the examples.
You can find more examples, where you have to use the linking "s" and where you should follow other rules.
Unfortunatly, the word "Nasenloch" (nostril) is an exception for this rule.
Have a look at the The Oxford Guide to Middle High German (MHG)