Where does “*उन्मग्न” come from?

*उन्मग्न (Sanskrit) comes from Sanskrit मग्न, from Proto-Indo-European mesg-nós.

Ancestry of “*उन्मग्न”, step by step

Step	Language	Word	Meaning
1	Sanskrit	मग्न	immersed
2	Proto-Indo-European	mesg-nós	—