From The Free On-line Dictionary of Computing (8 July 2008) [foldoc]:
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]
From Jargon File (4.4.4, 14 Aug 2003) [jargon]:
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.