Where does “Максим” come from?

Максим (Russian) comes from Old East Slavic Максимъ, from Old Church Slavonic Маѯимъ, from Ancient Greek Μάξιμος, from Latin Maximus, from Proto-Italic *magisVmos.

Ancestry of “Максим”, step by step

Step	Language	Word	Meaning
1	Old East Slavic	Максимъ	—
2	Old Church Slavonic	Маѯимъ	—
3	Ancient Greek	Μάξιμος	—
4	Latin	Maximus	the greatest
5	Proto-Italic	*magisVmos	—

Every word from Proto-Italic *magisVmos →

Максим — Etymology, Origin & Word History | Etymology Explorer