[Usenet; originally and more formally, net.kook] Term used to describe a regular poster who continually posts messages with no apparent grounding in reality. Different from a troll, which implies a sort of sly wink on the part of a poster who knows better, kooks really believe what they write, to the extent that they believe anything.
The kook trademark is paranoia and grandiosity. Kooks will often build up elaborate imaginary support structures, fake corporations and the like, and continue to act as if those things are real even after their falsity has been documented in public.
While they may appear harmless, and are usually filtered out by the other regular participants in a newsgroup of mailing list, they can still cause problems because the necessity for these measures is not immediately apparent to newcomers; there are several instances on record, for example, of journalists writing stories with quotes from kooks who caught them unaware.
An entertaining web page chronicling the activities of many notable kooks can be found at http://www.crank.net/usenet.html.