Where does “Πελοποννήσιος” come from?

Πελοποννήσιος (Ancient Greek) comes from Ancient Greek Πελοπόννησος, from Ancient Greek νῆσος — island.

Ancestry of “Πελοποννήσιος”, step by step

Step	Language	Word	Meaning
1	Ancient Greek	Πελοπόννησος	—
2	Ancient Greek	νῆσος	island

Πελοποννήσιος — Etymology, Origin & Word History | Etymology Explorer