Well I've managed to dig up something. "Coined in 1969 to describe a specific computer operating system, the term "Unix" now covers a whole host of variations, including Linux, FreeBSD, and Solaris. The name was intended as a pun on an earlier system called "Multics" (Multiplexed Information and Computing Service). This pun is the key to understanding the acronym, since the word "uniplexed" does not mean anything in particular. ("Multiplexed" refers to a communications system able to carry many messages simultaneously.)" - http://www.neoworks.com/info/technologies/glossary/
Wahey! Mystery solved.