I always made it two words. I heard someone (on the radio, I think) talking about some people, in pejorative lingo: "Just a bunch of Red necks." Having attended Mississippi State, I have at least some capacity for quipping on the term.
Usually, it is used in such a manner, i.e.., those of lesser degree, and usually from down South. However, a red neck is someone who has been working in the sun daily, to the point where the back of his neck is burned. Hard for some of us to wonder how a working man could be looked down on. But, I guess it takes all kinds.