Where does “tristis” come from?

tristis (Proto-Italic) comes from Proto-Indo-European tristh₂s, from Proto-Indo-European steh₂- — to stand.

Ancestry of “tristis”, step by step

Step	Language	Word	Meaning
1	Proto-Indo-European	tristh₂s	a third party standing, after the two parties to...
2	Proto-Indo-European	steh₂-	to stand

tristis — Etymology, Origin & Word History | Etymology Explorer