adj

• (music) About or inspired by science fiction, fantasy, horror, science, and/or subjects of interest to fans of speculative fiction; frequently, being a song whose lyrics have been altered to refer to science fiction; parodying. (However, much filk music is original rather than parodic.)
  "The blame/credit (choose one) for the first filk song is a little dubious. Like the man who tried to sit on two stools, it falls in the middle, between Poul Anderson who wrote a filk song called Barbarous Allen and Karen Anderson who egged him on and published it in Zed #774."

verb

• (intransitive) To perform filk music.
  "I could have filked all night"
• (intransitive) To participate in a filk circle, including singing along.
• (transitive) To write a parody of (a song).
  "1997 (?: "July A.S. XXXI") Medieval Melodies for Filking However, the practice of filking, of taking an existing melody and providing new, usually topical and/or satirical, lyrics, is in fact the direct counterpart of the Medieval practice of writing contrafacta."