Where does “Ústí” come from?

Ústí (Czech) comes from Old Czech ústie, from Proto-Slavic ustьje, from Proto-Slavic ustà, from Proto-Indo-European h₃éh₁os — mouth.

Ancestry of “Ústí”, step by step

Step	Language	Word	Meaning
1	Old Czech	ústie	—
2	Proto-Slavic	ustьje	—
3	Proto-Slavic	ustà	—
4	Proto-Indo-European	h₃éh₁os	mouth

Ústí — Etymology, Origin & Word History | Etymology Explorer