Moscow named after Moskva River
Etymology
The name of the city is thought to be derived from the name of the Moskva River. Several theories of the origin of the name of the river have been proposed. Finno-Ugric Merya and Muroma people, who were among the several pre-Slavic tribes which originally inhabited the area, called the river supposedly Mustajoki, in English: Black river. It has been suggested that the name of the city derives from this term.The most linguistically well-grounded and widely accepted is from the Proto-Balto-Slavic root *mŭzg-/muzg- from the Proto-Indo-European *meu- "wet", so the name Moskva might signify a river at a wetland or a marsh. Its cognates include Russian: музга, muzga "pool, puddle", Lithuanian: mazgoti and Latvian: mazgāt "to wash", Sanskrit: májjati "to drown", Latin: mergō "to dip, immerse". In many Slavic countries Moskov is a surname, most common in Russia, Bulgaria, Ukraine and North Macedonia. Additionally, there are similarly named places in Poland like Mozgawa.The original Old Russian form of the name is reconstructed as *Москы, *Mosky, hence it was one of a few Slavic ū-stem nouns. As with other nouns of that declension, it had been undergoing a morphological transformation at the early stage of the development of the language, as a result, the first written mentions in the 12th century were Московь, Moskovĭ (accusative case), Москви, Moskvi (locative case), Москвe/Москвѣ, Moskve/Moskvě (genitive case). From the latter forms came to the modern Russian name Москва, Moskva, which is a result of morphological generalization with the numerous Slavic ā-stem nouns.
However, the form Moskovĭ has left some traces in many other languages, including English: Moscow, German: Moskau, French: Moscou, Georgian: მოსკოვი, Latvian: Maskava, Bashkir: Мәскәү, Tatar: Mäskäw, Portuguese: Moscovo, Chuvash: Мускав, and Spanish: Moscú. 
In a similar manner the Latin name Moscovia has been formed, later it became a colloquial name for Russia used in Western Europe in the 16th–17th centuries. From it as well came English Muscovy and muscovite.Various other theories (of Celtic, Iranian, Caucasic origins), having little or no scientific ground, are now largely rejected by contemporary linguists.
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