I found this on whatis.techtarget.com :

fnord

Certain words are intended to be undefinable and "fnord" is one of them. First used in Robert Anton Wilson's trilogy, The Illuminati Papers, fnord has developed a group of devotees that meet at certain pages in cyberspace to celebrate the word's sense of the apparent but indefinite. The editors of whatis.com have spent far too much chasing the meaning of this elusive term, and this is what we are left with:

Fnord is the space between the pixels on your screen.
Fnord is the "ooo" in varooom of race cars.
Fnord is the smallest number greater than zero.
Fnord keeps a spare eyebrow in his pocket.

In Wilson's trilogy, (based on reports from users), truth is revealed to those who know where to look. The enlightened can see fnord in the empty spaces between unjustified columns of text in newspapers and magazines. Called sticky spots, these spaces are not really blank, but contain fnord, visible only to those privy to heavily guarded secrets and occult powers. Fans of the novel began using the word as a private joke and the concept spread to many who had never read the book but had no trouble discovering new places where fnord must clearly exist.