Where does “ustus” come from?

ustus (Latin) comes from Latin uro, from Proto-Italic ouzō, from Proto-Indo-European h₁ews- — to burn.

ustus (Latin): burnt, inflamed

Definitions

Step	Language	Word	Meaning
1	Latin	uro	I burn, consume, inflame; to rage, to ravage
2	Proto-Italic	ouzō	—
3	Proto-Indo-European	h₁ews-	to burn

ustus — Etymology, Origin & Word History | Etymology Explorer