Interesting thread. The Jargon file only says:
[In the authors' words, "A weak pun on Multics"; very early on it was
"UNICS"]
http://www.catb.org/jargon/html/U/Unix.html
It never occurred to me that the pun might not be recognized, even to
people whose first language is not English. Americans sometimes forget
that not everyone is American.