A phrase commonly used in (but not restricted to) baseball to describe something (usually a playing style or attitude) that is not respectable or sportsmanlike, but technically not breaking any rules of the game. Someone who commonly plays with antics that are considered bushleague, would be considered a "bushleaguer". Both "bushleague" and "bushleaguer" can also be used to describe unsportsmanlike acts by fans, and to describe the fans themselves, respectively.
BATTER: "Aww cmon ump, are you gonna let the catcher get away with that?? He keeps taunting me as the pitch comes in. This is such Bushleague crap.
UMPIRE: "I don't wanna hear any more of it. Now get in the box and shut your mouth."