BNC is somewhat short for Bouncer in the irc langugage. Its mostly used to get a nicer looking host or to hide yourself from something. They are also a good semi-replacement for irc robots, bots because some of the bouncers are on irc all the time, even when the original user has disconnected.

The popular irc bouncers include:

muh - http://mind.riot.org/muh/
psybnc - http://www.psychoid.lam3rz.de/
BNC - http://bnc.ircadmin.net/
ezbounce - http://druglord.freelsd.org/ezbounce/

(these links may vary with time)

Bouncers are offen run on shell servers. Some rootkits allso contain some version of psybnc.

BNC, Bayonet Neill-Concelman-type connectors (sometimes spelled out as Bayonet Nut, British Navy, Baby-N or Big Nobby Connector), named after the "Bayonet"-style of twist-on attachment, Paul Neill of Bell Labs (inventor of the N connector) and Carl Concelman (inventor of the C connector)

BNC is usually found at the end of coaxial cable used for the outdated 10Base2 chained ethernet networks, with semiprofessional video equipment and with high-speed measuring equipment like oscilloscopes. It has a superior degree of usability when compared to the more common kinds of plugs found with PC hardware.