What does Boxing Day have to do with boxing?
c.1400, "medicine prepared from mummy tissue," from Medieval Latin mumia, from Arabic mumiyah "embalmed body," from Persian mumiya "asphalt," from mum "wax." Sense of "embalmed body" first recorded in English 1610s. Mummy wheat (1842) was said to be cultivated from grains found in mummy-cases.
1784, childish alteration of mammy. Alternative form mumsy attested by 1876.