That 58 letter town on the Island of Anglesey
must be a real "bugger" to pronounce

The name was coined in the mid-19th century to provide the longest placename on record. The -fair- in the name means 'of Mary'. Llanfair means '(St) Mary's church'.

The phonological change of m to v is common in Celtic languages and is called lenition in linguistics (an older term was consonant mutation).

Ceci n'est pas un seing.