Naturally, the origins of folk MUSIC have been lost to time immemorial, that’s the whole nature of folk music.  however, there are origins of the TERM “folk music” which can undoubtedly be traced in the english language, and in the history of people thinking and writing about that music.  In every case, on every platform on which I looked for the term, there were references to it in ways that would be completely recognizable and consistent to refer to “folk music” to a modern listener–that is to say, in reference to the traditional music of a particular people, in opposition to music in the art tradition such as classical music.  Its first references are often to English minstrels, as well as the indigenous music of african-americans, the Irish, and Native americans. Indeed the first reference of any kind that I could find is in an 1805 edition of the magazine Music (https://books.google.com/books?id=rBGBQ3bHl5MC&pg=PA206&dq=%22folk+music%22&hl=en&sa=X&ved=0CDEQ6AEwBDgyahUKEwi91qO8p_DIAhWLaD4KHXuNDuU#v=onepage&q=%22folk%20music%22&f=false), found through google books and n-grams alike, where the reference is to the music of the Omaha Indians, described as belonging “aesthetically…to the primitive origins of melody, proceeding along harmonic lines” and noting that “from an ethnological point of view, these melodies are interesting”.  Similarly we see in the Philaelphia Evening Telegraph in 1870 (http://search.proquest.com/news/docview/171377199/260E886B50CE4739PQ/1?accountid=14541) that “Scotland and Germany stand pre-eminent for folk-music”, a term which is used interchangeably with “people’s music”, hinting at the german origin of the term in the word “volk”, meaning “common people”.  The article also notes that the significance of folk music in the cultures it looks at is two-fold: “not only the number, richness and beauty of these songs, but their present vitality in their fatherlands”.  There are references in an Atlantic Monlthy article from 1873 (http://search.proquest.com/news/docview/171377199/260E886B50CE4739PQ/1?accountid=14541) to Chaucer and how his works are imbued with folk-song traditions, showing clearly just how far back these traditions were recognized to go, even from the earliest days of discussing them. These references makes plain three things: One, that the term folk music is used not only to describe the music itself and how it sounds but also to ground that music in a particular culture and highlight that its study is essential to the study of that culture. Two, that the term “primitive” was clearly less troubling to use in the 19th century than it is today to refer to non-western cultures. Three, that the term “folk music” was probably in use much longer than the time period covered by the databases available, as it’s difficult to see any “evolution” of the term’s use.  Every reference to it is in a recognizable use, the one you’d expect to see going in to this blind.  If I had to guess, I’d say that using the term “folk music” to refer to this traditional, ethnic-cultural music which has value in study not only culturally but anthropologically, and which is defined in opposition to the formal tradition of art music, dates at least back to the 18th Century, and given the resources to study documents from further back, I’d be interested to see the genesis of the term, which clearly predates the available sources in these databases, as I doubt the term emerged fully-formed like Athena from its very first reference.