The Jargon File (version 4.4.7, 29 Dec 2003):

filk
 /filk/, n.,v.

    [from SF fandom, where a typo for ?folk? was adopted as a new word]
    Originally, a popular or folk song with lyrics revised or completely new
    lyrics and/or music, intended for humorous effect when read, and/or to be
    sung late at night at SF conventions. More recently (especially since the
    late 1980s), filk has come to include a great deal of originally-composed
    music on SFnal or fantasy themes and a range of moods wider than simple
    parody or humor. Worthy of mention here because there is a flourishing
    subgenre of filks called computer filks, written by hackers and often
    containing rather sophisticated technical humor. See double bucky for an
    example. Compare grilf, hing, pr0n, and newsfroup.


The Free On-line Dictionary of Computing (30 December 2018):

filk

   /filk/ [SF fandom, where a typo for "folk" was adopted as a
   new word] A popular or folk song with lyrics revised or
   completely new lyrics, intended for humorous effect when
   read, and/or to be sung late at night at SF conventions.
   There is a flourishing subgenre of these called "computer
   filks", written by hackers and often containing rather
   sophisticated technical humour.  See double bucky for an
   example.  Compare grilf, hing and newsfroup.

   [Jargon File]