The shady underground world of the Internet where most seriously illegal activities are conducted. The Undernet is the "underground of the Internet".
Phishers steal identification data from unsuspecting people and then sell that data on the Undernet, which among other things contains this huge black market operating under the Internet where all kinds of ill-gotten data are bought and sold. The seeds for identity theft are sown on the Undernet.
by Internet Veteran April 26, 2012
Top Definition
One of the largest IRC networks still in existence with a laid back administration that provides minimal services but doesn't overwhelm the users with hundreds of *Serv bots. The primary service is X, the channel bot, who provides many options for managers and ops alike.

Undernet is very involved with its users, and holds frequent classes and interesting discussions or interviews in the #class channel. Channels are very hard to register, and therefore if a channel is registered, then it is garunteed to be lively. It has frequent netsplits, but not quite to the caliber of EFnet and DALnet, making it a much more solid network. In general, the help channels on Undernet are often nicer and more humble than EFnet, but still don't be surprised if you get banned for an arbitrary reason. There's always room for improvement.
<Varter> Hey, wanna chat on IRC instead?
<Mancow> Sure, I'll meet you in #tech on Undernet.
<Varter> Nice.
by Varter August 27, 2004
A person to person network being setup to have insurance aganst the internet being shut down.
I just sent a copy of some of the urban dictionary into the undernet
by Jerryu June 08, 2010
The vast, insidious and unstable domain of the Internet home to the worst depravities and ideals of mankind. Hidden behind the vail of the "Polite Internet," the Undernet is host to the worst and most scarring images ever available to humanity, as well as opinions and ideas from individuals that minds have been cracked and malformed by visits to the Undernet. Self propagating, the Undernet is the fastest growing part of the Internet, and despite the best efforts of the government, grows unchecked. The best way to describe the Undernet is as "online urban blight" or "cyber-pestilence."
Mark: "Hey, check out this website"
Tom: "Okay...holy god in heaven why?"
Mark: "What, it is just German's peeing on babies while driving spikes through their penises?"
Tom: " need a break from the Undernet"
by Jay Krank July 29, 2004
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