Where does “Усть-Луга” come from?

Усть-Луга (Russian) comes from Russian у́стье, from Proto-Slavic ustьje, from Proto-Slavic ustà, from Proto-Indo-European h₃éh₁os — mouth.

Ancestry of “Усть-Луга”, step by step

Step	Language	Word	Meaning
1	Russian	у́стье	—
2	Proto-Slavic	ustьje	—
3	Proto-Slavic	ustà	—
4	Proto-Indo-European	h₃éh₁os	mouth

Усть-Луга — Etymology, Origin & Word History | Etymology Explorer