A metasyntactic variable, similar to foo and bar.
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 Illuminatus! trilogy.
The word fnord is a modern invention that originated in the Principia Discordia. It later became widely known through its appearance in the Illuminatus! trilogy.