Brockhaus and Efron Encyclopedic Dictionary
Its first element is most likely the Indo-Iranian "bhaga" "god, lord" (c.f. bey). A suggestion cited in the "Brockhaus and Efron Encyclopedic Dictionary" derives the word from Sanskrit "baghadhara". Alternatively, Gerard Clauson suggests that "bağatur" was in origin a Hunnic proper name, specifically that of Modu Chanyu.