is ' terrestrial ' related to ' teres '

Welcome to the board, learntm. Not sure I understand your second word (do you mean the medical term?), but terrestrial derives from the latin terra meaning earth.

One suspects that the teres muscles derive their name from "tero" -- the Latin verb meaning "to rub."

I think learntm is referring to the latin root of today's word:

>>[From Latin teret-, stem of teres (round).]<<

Welcome, tm!