Where does “synonymus” come from?

synonymus (Latin) comes from Ancient Greek συνώνυμος, from Ancient Greek σῠν-, from Ancient Greek σύν, from Proto-Hellenic *ksún — beside, with.

Ancestry of “synonymus”, step by step

Step	Language	Word	Meaning
1	Ancient Greek	συνώνυμος	Having the same name; Having only one meaning:...
2	Ancient Greek	σῠν-	together; with, together
3	Ancient Greek	σύν	beside, with
4	Proto-Hellenic	*ksún	beside, with