Acronym for All Knowledge Is Contained In Fanzines. The phrase, introduced by Charles Burbee, is tongue-in-cheek. At present, to make it a little more egalitarian, it is sometimes rendered as All Knowledge Is Contained In Fandom.
As initials, the tag first appeared and is used mainly in rasff, where it generally indicates that a poster is asking a question that has nothing to do with SF or fandom. (The initials are clearly post-1950s; the original phrase may have been an aphorism of Charles Burbee’s.)