Introduction. The article examines the onym Megüren (Yak. Möŋürüön < Mögürüön) used as a name of several administrative units in the territory of Yakutia, mainly those included in Meginsky (Yak. Mäŋä) District. The available 17th-century written sources — i.e. earliest Russian-language documents on Yakuts — mention no such onym. And it was E. Pekarsky who already pointed out that the Yakut word mögürüön ‘round-thick’ could be a Mongolic borrowing and, in particular, tended to trace parallels in the Buryat language. Subsequent researchers paid no attention to both the word and the corresponding ethnonym. Goals. The paper aims to analyze origins of the word the ethnic name stems from. Materials and methods. In the absence of early historical accounts, the work explores linguistic sources to investigate phonetic appearances of the Yakut onym in question and comparable data in other languages, primarily Mongolic ones. The latter include not only vocabularies but also materials dealing with personal onomastics. Some folklore elements also prove instrumental in settling the issue. Conclusions. The analysis of phonetic properties inherent to the Yakut ethnonym möŋürüön — in comparison with different forms of the word in Buryat dialects — makes it possible to conclude that it penetrated the Yakut discourse precisely from a language essentially close to western Buryat dialects characterized by the use of /ö/ in the first syllable (/ü/ in standard Buryat) and vowel labialization in non-first syllables. Other features outline the upper chronological limit of the word’s arrival in Yakut to the late 17th century since the observed properties are as follows: /g/ > /ŋ/ assimilation; presence of a long vowel in -VgV- complex, and the intervocalic /g/ from the Mongolic /k/ not yet transformed into the Buryat /χ/.