A jock is a young male whose entire life revolves around proving that he is superior to everone else, hence, the involvement in competitive sports. Jocks are trained to trample on other people without feeling any remorse. Some females can be jocks, too. The important thing is NOT that they are actually good at sports, but that they ernestly believe that they are superior and that they act that way. They, of course, have their circle of friends (as many as possible, so as to show everyone how "friendly" they are and to make themselves feel good) who they do acknowledge as nearly their equals. But anybody else is dirt beneath their feet and not worthy to even speak to them.
You see them everywhere. Some stop being jocks after high school, when they find out they really weren't that great after all, even in their chosen sport. Others remain jocks throughout college, or welding school, or whatever. Some remain jocks into their thirties and beyond. By then, it has nothing to do with sports. It's all in the attitude as, of course, it always was.