"Unix" is not an acronym, so it doesn't stand for anything. A a contrast to
an earlier operating system, Multics, Brian Kernighan coined the word to
indicate that "Unix" is simpler and more unified. In fact, the Unix system
is Multics without balls.
Unix /yoo'niks/ n.
[In the authors' words, "A weak pun
on Multic very early on it was `UNICS'] (also `UNIX') An
interactive time-sharing system invented in 1969 by Ken Thompson
after Bell Labs left the Multics project, originally so he could
play games on his scavenged PDP-7....