I had the same question, but then came to my own conclusion after reading as much as I could from a variety of venues, as well as other people's comments, thoughts, etc. - and a little common sense while taking the old time way of life & other things that existed a century ago (or so), amongst the 'mountain people' of the 'South'.
So my own interpretation boils down to a good ol', hardcore, more or less isolated (almost to the degree of a hermit), totally self-sufficient, super rugged, grizzly wrestling "Mountain Man".

Also way back when, a JACK was typically used as a term referring to a hardy 'manly man' of sorts - just as in the WWII Era, a man in general was referred to as 'Joe'... (Hey Joe, gotta a light?) BUT in the case of 'Jack', we're going back even further to a time when people were pioneers homesteading, trying to cut out a life for themselves, in this case - high up, deep in the mountains (Appalachian/ Smokey/ Blue Ridge/ Rocky/ etc.)