What does "dunker" mean?

Dunker: 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. Note: The denomination was founded in Germany in 1708, but after a few years the members emigrated to the United States. 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.