I love my "Justa" and prefer calling it that to calling it a "non-S," which makes it sound likes it's missing something...Most of the time I just call it a Mini since most non-Mini people don't know the difference anyway.

 

While I wasn't around when the term originated I don't think it came about in a mean spirited way. If I had to guess I would say it probably originated with people introducing themselves on the forum. Something like this:

New Owner: "I'm so stoked!, I got a MINI Cooper today!"
Forum Member: "Awesome dude! What color is it? Is it an S or a non-S, post some pictures when you get a chance."
New Owner: "It's just a Cooper, Chili Red, with the sport package, white wheels......"