This is a difficult question to answer as is, since the definition of the word "filk" itself is somewhat nebulous. It is easier, however, to pinpoint the origin of the term "filk". According to several sources, the term came from a typo. In her essay on the history of filking (see references at the end of this article), Lee Gold points out that the typo was made by a fan submitting an essay to SAPS (Spectator Amateur Press Society). In his title, ""The Influence of Science Fiction on Modern American Filk Music", the word "Filk" was originally supposed to be "Folk". The essay was rejected, but word spread about the typo, and the term stuck.

Additional references

An Egocentric and Convoluted History of Early "Filk" And Filking by Lee Gold. History of Filk (from Wikipedia entry on Filk Music) History of Filk in Germany and Austria by Juliane Honisch. A Brief History of (British) Filk by Rhodri James -- Ed. note: couldn't get through on April 7th/2007 Filklore and filklife studies by Jed Hartman (1980), with some modifications. Includes history of filk publishing.