word-forming element meaning "together, with," sometimes merely intensive; it is the form of com- used in Latin before consonants except -b-, -p-, -l-, -m-, or -r-. In native English formations (such as costar), co- tends to be used where Latin would use con-.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit tarsayati "dries up;" Avestan tarshu- "dry, solid;" Greek teresesthai "to become or be dry," tersainein "to make dry;" Latin torrere "dry up, parch," terra "earth, land;" Gothic þaursus "dry, barren," Old High German thurri, German dürr, Old English þyrre "dry;" Old English þurstig "thirsty."
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<a href="https://www.etymonline.com/word/conterraneous">Etymology of conterraneous by etymonline</a>
Harper, D. (n.d.). Etymology of conterraneous. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/conterraneous