Green’s Dictionary of Slang

anywhere considered second-rate, out-of-the way.

1909		Eve. Star (Wash., DC) 17 Oct. 5/3: A young pitcher [...] was resh from the ‘Bush League’ and it was his first appearance at a ‘Big League’ hotel.
1912		Ade Knocking the Neighbors 106: [She] would have to make an immediate Demonstration of Form to avoid being permanently Benched or sent back to the Bush League.
1935		(con. 1920s) J.T. Farrell Judgement Day in Studs Lonigan (1936) 617: The bush leagues for you, Spunk [...] You’re getting old.
1935		C. Odets Awake and Sing! II i: Don’t be a bush leaguer all your life.
1944		Reader’s Digest Oct. 55/1: They have left the political bush leagues and are now in the majors [DA].
1955		B. Schulberg On the Waterfront (1964) 310: Looks like I’m being sent down to the bush leagues.
1969		L. Schecter Jocks 115: [H]e signed an article in Sports Illustrated (for about 15,000 dollars) titled ‘My life in a Bush League’.