Where does “terstis” come from?

terstis (Latin) comes from Latin trīstis, from Proto-Indo-European tristh₂s, from Proto-Indo-European steh₂- — to stand.

Ancestry of “terstis”, step by step

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

terstis — Etymology, Origin & Word History | Etymology Explorer