Complex sentence: The name was originally Tabgach (as attested to in the Orhun inscriptions in the Göktürk language), and most likely meant "earth-origined" (as in "toprak"), or "round" (as in "Topkapi") in Turkish.
Split into simpler sentences: The name was originally Tabgach, as attested in the Orhun inscriptions in the Göktürk language. It most likely meant "earth-origined" (as in "toprak"), or "round" (as in "Topkapi") in Turkish.