Wiktionary
interj. 1 ''(http://en.wikipedia.org/wiki/Discordianism)'' A word (which may or may not be "fnord" itself) commonly held to be invisible to the conscious mind, but subliminally causing a sense of unease or sudden anger when encountered. So used in the http://en.wikipedia.org/wiki/Illuminatus%20trilogy. 2 ''(http://en.wikipedia.org/wiki/Discordianism)'' A word defined as having no definition. n. A metasyntactic variable, similar to foo and bar.
Wikipedia
Fnord is a word that was coined in 1965 by Kerry Thornley and Greg Hill in the Principia Discordia. It entered the popular culture after appearing in The Illuminatus! Trilogy (1975) of satirical and parody conspiracy fiction novels by Robert Shea and Robert Anton Wilson. In these novels, the interjection "fnord" is given hypnotic power over the unenlightened, and children in grade school are taught to be unable to consciously see the word "fnord". For the rest of their lives, every appearance of the word subconsciously generates a feeling of uneasiness and confusion, and prevents rational consideration of the subject.
The word is often used in newsgroup and hacker culture to indicate that someone is being ironic, humorous or surreal. Often placed at the end of a statement in brackets (fnord) to make the ironic purpose clear, it is a label that may be applied to any random or surreal sentence, coercive subtext, or anything jarringly out of context (intentionally or not). It is sometimes used as a metasyntactic variable in programming. It appears in the Church of the SubGenius recruitment film Arise! and has been used in the SubGenius newsgroup alt.slack.
Usage examples of "fnord".
It was the New York Times editorial page, and not a picture on it, so he certainly didn't see the fnords, whatever the hell they are, there.
Let me be very clear and precise about this, since many of the people on this trip are deliberately and perversely obscure: I would not, could not, have seen the fnords if Hagbard Celine hadn't hypnotized me the night before, on the flying saucer.
The fnords produced a whole population walking around in chronic low-grade emergency, tormented by ulcers, dizzy spells, nightmares, heart palpitations and all the other symptoms of too much adrenalin.
If I pointed out a fnord to somebody who hadn't been de-conditioned, as Hagbard deconditioned me, what would he or she say?
He had become painfully conscious of them since Hagbard had removed the aversion reflex, and each fnord had sent a pang through him that was a ghost of the low-grade emergency in which he had previously lived.
I made breakfast in a pretty nasty mood, wondering if I'd seen the fnords, whatever the hell they were, in the hours he had blacked out, or if I would see them as soon as I went out in the street.