*n?x command to kill every process owned by the user who typed it, except for the login shell. This is useful for dealing with programs that have frozen and hung the terminal, or to fix a fork bomb.
This is (obviously) a dumb thing to do as root; because root owns all processes, kill -9 -1 will effectively DoS the entire system. It also can cause trouble when mixed with X sessions, leaving the console in an unusable state, but the latter can be fixed with telnet if the computer is online.
BOFH's may regard this as a useful command to suggest lusers to put in their .profile.