What does Boxing Day have to do with boxing?
1530s, "one who baptizes over again," from Modern Latin anabaptista, from Latin anabaptismus "second baptism" (used in literal sense from 4c.; see anabaptism).
Originally in English in reference to sect that practiced adult baptism and arose in Germany 1521. Probably so called because, as a new faith, they baptized converts who already had been baptized (as infants) in the older Christian churches. Modern branches only baptize once (adults) and do not actively seek converts. The name also was applied, usually opprobriously, to Baptists, perhaps due to the multiple immersions of their baptisms.