Im not sure what the others think it means. A plug ugly is a young (typically) boy that used to take a bucket and put it over the wooden fire plugs that would be tapped into the hollowed out logs that used to be our water supply back when fire departments were actually just insurance companies. We are talking back in the Ben Frank days. Things were fiercely competitive back then, often times ending in street brawls. Needless to say, the plug ugly's were paid by competing insurance companies to hide the water supply, so the other insurance company would not be able to extinguish the fire.
Its a name I adopted many years ago on some 4 wheeling forums, and it has just stuck.