The name Fenner is in orignal Venn, which describes Hochmoor (peat bog). It means three waste (and not longer existing) towns in the country Maden, Ritter-, Mittel- und Langenvenne. Maden is today a town in Gudensberg. The other named towns are in the west of Gudensberg in the near of the highway A49.
These three towns are first seen in a document from 1045. This document contains the donation of the country to Chuniza and the renaming to Vanaha. It is believed that the name Venne was born now. Later it is written Venner.
In 1370 the first Fenners were seen in Ziegenhain and Treysa (Schwalmstadt) between Fritzlar and Alsfeld. From here are the most Fenners in Germany. There are Fenners in Tirol, Swiss and England, too. But the relationship between those and the Fenners in Schwalmstadt is not secure.
It is possible that the name Fenner in England is belong to Venn (english fen) and in Swiss to Fähnrich (flag bearer).