A fannish folksong, often a parody of standard folksongs. Also: filksong. (2) A filksong festival. Also filkfest. (3) filk: To sing filksongs. Also: filksinging.
Sounds dirty, but this simply refers to a parody or rewriting of a song to cater to a specific fandom. It also refers to any original song written about or within a particular fandom.
a new lyric set to an existing tune