According to the late Franz Joseph in his USS Enterprise blueprints, it was "Naval Construction Contract". Later one of the unlicenced fan technical books (Ships of the Star Fleet?) used the term "Navigational Contact Code" which has a nice ring to it.
NX was assumed to be experimental until Star Trek: Enterprise
established it originally as a starship class designation, like DY or J-class. I think now it just means "ships to cool for C's"