A k-line or kill line is an IRC term. When a user is k-lined, he or she is banned from a certain server, either for some time or forever. When the user is banned they are not allowed to join that server again and have to join another one to get back onto IRC. This is recorded as a line in the server's IRC daemon configuration file which has the letter "K" at the beginning. This is where "K-line" comes from.