This is the 8338th most frequent Russian word.
митрошиной
The word "митрошиной" does not appear to have a standard meaning in Russian; it may be a surname or a specific term requiring additional context.
Here, 'Митрошиной' is used as the possessive form of a surname, indicating belonging or identity.
Она гордилась своей фамилией Митрошиной.
She was proud of her surname Mitroshina.
In this context, 'Митрошиной' is an instrumental case of a proper noun, used to indicate the means or accompaniment of the action.
Я встретился с Митрошиной на конференции.
I met with Mitroshina at the conference.
Here, 'Митрошиной' comes as a possessive referring to the person associated with the book, functioning as an adjective.