Where does “Σέρρες” come from?

Σέρρες (Greek) comes from Byzantine Greek Σερραί, from Ancient Greek Σίρις.

Ancestry of “Σέρρες”, step by step

Step	Language	Word	Meaning
1	Byzantine Greek	Σερραί	—
2	Ancient Greek	Σίρις	—

Σέρρες — Etymology, Origin & Word History | Etymology Explorer