Where does “ἑξήντα” come from?

ἑξήντα (Byzantine Greek) comes from Ancient Greek ἑξήκοντα, from Proto-Indo-European sweḱsḱomth₂, from Proto-Indo-European swéḱs-dḱomt-h₂ — six-ten.

Ancestry of “ἑξήντα”, step by step

Step	Language	Word	Meaning
1	Ancient Greek	ἑξήκοντα	sixty; a college of γελωτοποιοί at Athens
2	Proto-Indo-European	sweḱsḱomth₂	—
3	Proto-Indo-European	swéḱs-dḱomt-h₂	six-ten

ἑξήντα — Etymology, Origin & Word History | Etymology Explorer