/ku̇-kē män(t)-stər/
[from Sesame Street]
n. Any of a family of early (1970s) hacks reported on TOPS-10, ITS, Multics, and elsewhere that would lock up either the victim's terminal (on a time-sharing machine) or the console (on a batch mainframe), repeatedly demanding "I WANT A COOKIE". The required responses ranged in complexity from "COOKIE" through "HAVE A COOKIE" and upward.
See also wabbit.