It's derived from the Russian word мышь (mysh), meaning "mouse". So, Myshkina could be interpreted as "daughter of the mouse" or someone who is associated with mice in some way.
However, without further context, it's impossible to give a precise meaning. It could be a last name, a nickname, or even a fictional character.
Could you provide more information about where you encountered this word? That would help me give you a more accurate answer.