Where does “terstus” come from?

terstus (Latin) comes from Latin tersa, from Latin tersus, from Latin tergo, from Proto-Indo-European terh₁- — to rub, turn; to drill, pierce.

Ancestry of “terstus”, step by step

Step	Language	Word	Meaning
1	Latin	tersa	nominative feminine singular of tersus;...
2	Latin	tersus	clean, neat, rubbed or wiped , cleansed, having...
3	Latin	tergo	Alternative form of tergeō; ablative singular of...
4	Proto-Indo-European	terh₁-	to rub, turn; to drill, pierce

terstus — Etymology, Origin & Word History | Etymology Explorer