The original Old Russian form of the name is reconstructed as *, *, 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 ,  (accusative case), ,  (locative case), ,  (genitive case). From the latter forms came the modern Russian name , , which is a result of morphological generalisation with the numerous Slavic ''ā''-stem nouns.