Where does “мио” come from?

мио (Serbo-Croatian) comes from Proto-Slavic milъ, from Proto-Balto-Slavic meiʔlos, from Proto-Indo-European meyl-.

Ancestry of “мио”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	milъ	dear, nice, gentle, mild, soft, pleasant, polite
2	Proto-Balto-Slavic	meiʔlos	—
3	Proto-Indo-European	meyl-	—

мио — Etymology, Origin & Word History | Etymology Explorer