Majewski, is a name that was used by Jewsh families who'd converted to Catholicism.
This is partly true for the members of the Jewish religious movement, Frankism, set up by Jacob Frank (1726-1791), being in opposition to traditional Judaism. Of the Frankists who remained in Poland after 1772 (24,000 people), the majority of those who converted to Catholicism in Warsaw in 1795 adopted the surname Majewski since the act took place in May (in May of 1795 the Polish capital was already under Prussian rule); I have no idea of their exact number, however, but I'm sure it must have been established by the historians.
Otherwise, Majewski is a purely Slavic surname. It is also worth noting that the Jews who were baptized in Poland often received the surnames of the nobel men who were "adopting" those Jewish families, for example, Branicki, Baranowski.