1. An adjective referring to anything sub-par, or of a lesser or lower grade.
2. An adjective meaning bullshit or bunk.
The term is derived from Nascar's joke of an undercard, the Busch League of racing. Not only is the league of a lesser grade than Nascar, the whole idea of racing as a sport is bunk.
Jon: What's up baby girl? You tryin' to bone.
Jane: Please, homeboy. Your game is Busch League.