Anabaptist (n.)

class of Christians who regard infant baptism as invalid, 1530s, literally "one who baptizes over again," from Modern Latin anabaptista, from Late Latin anabaptismus "second baptism" (used in literal sense from 4c.), from Ecclesiastical Greek anabaptismos, from ana "again, anew" (see ana-) + baptismos "baptism" (see baptism).

Originally in English in reference to the sects that practiced adult baptism and arose in Germany from 1521. Probably so called because, as a new faith, they baptized converts who already had been baptized (as infants) in the older Catholic or older Protestant churches. Modern branches (notably Mennonites and Amish) baptize only 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.