The Collaborative International Dictionary of English v.0.48:

Dunker \Dun"ker\, prop. n. [G. tunken to dip.]
   One of a religious denomination whose tenets and practices
   are mainly those of the Baptists, but partly those of the
   Quakers; -- called also Tunkers, Dunkards, Dippers,
   and, by themselves, Brethren, and German Baptists, and
   they call their denomination the Church of the Brethren.
   [1913 Webster]

   Note: The denomination was founded in Germany in 1708, but
         after a few years the members emigrated to the United
         States; they were opposed to military service and
         taking legal oaths, and practiced trine immersion.
         [1913 Webster + WordNet 1.5]

   Seventh-day Dunkers, a sect which separated from the
      Dunkers and formed a community, in 1728. They keep the
      seventh day or Saturday as the Sabbath.
      [1913 Webster]