The Oxford English Dictionary refers to it as a nonce-word (i.e. coined for just one occasion), meaning 'belonging to or characteristic of the people', and cites the source as 'The angler in Wales; or, Days and nights of sportsmen' by Thomas Medwin, 1834. ("Perhaps beauty is demic or epidemic here").
Whether your hearing of it is related to the origin and meaning is, of course, unsure. There might be a similar dialect word (with a completely different meaning) that has, as yet, escaped the attention of the OED's compilers.