Douglas Harper's Etymology Dictionary
bush league
"mean, petty, unprofessional," 1906, from baseball slang for the small-town baseball clubs below the minor league where talent was developed (by 1903), from bush (n.) in the slang sense of "rural, provincial," which originally was not a value judgment.
Wiktionary
bush league
a. 1 Having inferior quality. 2 Amateurish. n. 1 (context chiefly US idiomatic English) A professional sports association at the lower levels of minor league organization. 2 (context figuratively by extension English) A low-ranking or inferior level among groups, professions, organizations, etc.
WordNet
bush league
n. a league of teams that do not belong to a major league (especially baseball) [syn: minor league, minors]