As a historian, Edwards was interested in preeserving some anecdotes, chronologies and facts which would otherwise have perished. Since he understood believer's baptism to be a root principle of Christianity, he held this in common with the Brethren although he also noted beliefs which he did not share with the Brethren. Edwards also wrote about the Mennonites and often distinguished them from the Brethren whom he referred to as "Tunkers."
They are called Tunkers in derision which is as much as to say Sops, from tunken, to "put a morsel into sauce;" but as the term signifies Dippers they may rest content with the nickname, since it is the fate of Baptists in all countries to bear some cross or other. They are also called Tumblers, from the manner in which they perform baptism, which is by putting the party's head forward under water (while kneeling) so as to resemble the motion of the body in the action of tumbling. The Germans sound the letters t and b like d and p; hence the words Tunkers and Tumblers have been corruptly written Dunkers and Dumplers.
Source: The Brethren in Colonial America, Donald F. Durnbaugh