A bush league is slang in the vernacular of the U.S. for a minor league usually referring to smaller sports leagues. It is also used to describe things other than sports leagues as small, inferior, unprofessional, tacky, or trivial.

Used in The Big Lebowski when "The" Jesus describes Walter's wielding of a firearm in a bowling tournament as "bush league psycho stuff."

Fun fact: Webster 1913 doesn't cover this one becuause it was added to Webster's dictionary in 1914.