Medieval Latin, from Bulgari "Bulgarians," perhaps literally "the men from the Bolg," the River Volga, upon whose banks they lived until 6c. But the people's name for themselves in Old Bulgarian was Blugarinu, according to OED, which suggests a different origin. In other sources [such as Room], the name is said to be ultimately from Turkic bulga "mixed," in reference to the nature of this people of Turko-Finnish extraction but Slavic language.