[from SF fandom, where a typo for 'folk' was adopted as a new word]
n.,v. A 'filk' is 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 humor.
See double bucky for an example.