Where I grew up on the edge of the the fens in East Angla, It was a derogatory slur that the people who came from the reclaimed agricultural flatlands of the fens ( parts of Cambridgeshire, Norfolk, Lincolnshire) were a little bit backwards. The accent changes quite markedly from the fenland area as soon as you leave it.
The bog dwelling reed eating 6 toed hunchback with a gradient allergy and a bearded sister/wife/mum who drove you to school in a tractor with a bit of a Norfolk accent was the general stereotype.
If you lived in the fens, you were a "Fenner".