A clipped transliteration of "Augustus" — the full Chinese form for the Roman emperor is usually
奥古斯都 (Àogǔsīdū).
奥古斯 by itself shows up in subtitles for shorter names like "August" or as a partial reading. Each character is purely phonetic:
奥 (ào) = "Au",
古 (gǔ) = "gu",
斯 (sī) = "s".