Wikipedia
Mummerset refers to a fictional rustic English county, and more commonly, the English dialect supposedly spoken there. Mummerset is used by actors to represent a stereotypical English West Country accent while not being specific to any actual county.
The name is a portmanteau of mummer (an archaic term for a folk actor) and Somerset, a largely rural county.
Mummerset draws on a mixture of characteristics of real dialects from the West Country, such as rhoticism, forward-shifted diphthongs, lengthened vowels, and voicing of word-initial consonants that are voiceless in other English dialects. Word-initial "S" is replaced with "Z"; "F" is replaced with "V". It also uses perceived dialect grammar, replacing instances of "am", "are" and "is" with "be". The sentence "I haven't seen him, that farmer, since Friday" could be parsed in Mummerset as "Oi ain't zeen 'im that be varmer since Vroiday".