There are two theories about the name.
There are lots of theories but none of them proven or even convincing enough. Another popular one derives the name from the bear, niedźwiedź (miś). Other possibilities are a purse (miech, mieszek) or a diminutive from two-part names like Kazimierz
. Nobody really knows and there's a lot of discussion on it as it was the name of the first historical ruler of Poland.
A sample discussion on a historical forum - unfortunately, only in Polish: