A "blue bomb" (also known as "WinNuke") is a technique for causing the Windows operating system of someone you're communicating with to crash or suddenly terminate. The "blue bomb" is actually an out-of-band network packet containing information that the operating system can't process. This condition causes the operating system to "crash" or terminate prematurely. The operating system can usually be restarted without any permanent damage other than possible loss of unsaved data when you crashed.
The blue bomb derives its name from the effect it sometimes causes on the display as the operating system is terminating - a white-on-blue error screen that is commonly known as blue screen of death. Blue bombs are sometimes sent by multi-player game participants who are about to lose or users of Internet Relay Chat (IRC) who are making a final comment. This is known as "nuking" someone. A commonly-used program for causing the blue bomb is WinNuke. Many Internet service providers are filtering out the packets so they don't reach users.