Noun; is used to describe a man who is very manly and does dude stuff all the time. Being referred to as a fifty-niner requires a current fifty-niner to give you such a title. Membership includes lifting, eating much, and growing a mustache. Only dudes who do dude shit in a dudely way can be a fifty-niner.
Steven Segal, Bruce Willis, and Jean Claude Van Dame are fifty-niners