Medieval Latin, from Bulgari "Bulgarians," traditionally explained as "the men from the Bolg," the River Volga, upon whose banks they lived until 6c. But evidence is wanting, and the people's name for themselves in Old Bulgarian was Blugarinu, according to OED and Century Dictionary, 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.
variant of suffix -an (q.v.), with connective -i-. From Latin -ianus, in which the -i- originally was from the stem of the word being attached but later came to be felt as connective. In Middle English frequently it was -ien, via French.
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Definitions of Bulgarian from WordNet
a native or inhabitant of Bulgaria;
a Slavic language spoken in Bulgaria;
of or relating to or characteristic of Bulgaria or its people;