Where does “mild-n-” come from?

mild-n- (Proto-Balto-Slavic) comes from Proto-Indo-European ml̥dʰ-n-.

Ancestry of “mild-n-”, step by step

Step	Language	Word	Meaning
1	Proto-Indo-European	ml̥dʰ-n-	—

mild-n- — Etymology, Origin & Word History | Etymology Explorer