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.
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.