IOS is really a stripped-down Unix-variant (BSD). You know, Unix, the world's most ubiquitous operating system, that supports multiple user accounts out of the box.
And whose name, in fact, stems from "uni-plexing" -- it was an operating system designed to support two users, so that Dennis and Ken could play Space Travel. A weak pun on "Multics" (multiplexing operating system).
http://cm.bell-labs.com/who/dmr/hist.html