What is the origin of the phrase "bush league"?

Solution 1:

According to Etymonline,

"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.

In other words, we're speaking not of bushes, but of the bush:

1) an uncultivated or sparsely settled area… 2) the countryside, as opposed to the city.

Americans do not commonly refer to rural areas as the bush these days (though Australians and South Africans do), but we do speak of the sticks, the boondocks, and the backcountry among others— "places" which are in the middle of nowhere, and as a result whose natives lack refinement in etiquette or skills.

Bush remains in baseball, but it seems only as a pejorative.

Solution 2:

Here is the entry for "bush league" in Paul Dickson, The Dickson Baseball Encyclopedia (1989):

bush league 1. n. Lesser minor league teams in small cities or towns, such as when a player has slipped from "the big leagues to the bush" or one who has "just come up from the bush league."

ETY[MOLOGY:] From the nickname for the lower levels of the minor leagues, which were traditionally typified as out where the bushes grow, where all the land has not been cleared.

2. adj. BUSH [that is, "unprofessional, unsportsmanlike, amateurish"]

1ST [OCCURRENCE] 1908. (Sporting Life, February 10)

As noted in choster's answer, Etymology Online traces "bush league" in the sense of small-town baseball clubs to the year 1903. However, an Elephind newspaper database search for "bush league" turns up several instances of the term from the turn of the century. First, from "Ten on the Ground," in the Kansas City [Missouri] Journal (April 4, 1899):

"Notwithstanding the weather conditions that have prevailed for the past twenty-four hours, several of the Blues reported for the regular spring practice yesterday morning," said Old Sport last night. I was on the ground myself, but I can give you every assurance that the snowy sheet that covered Manning's lot was not marred in its virgin whiteness by the cruel foot of any member of the coming champions. The boys passed the time telling pipe dreams of the past winter and the contracts they were offered by the Bush league, but were prevented from accepting on account of their filial affections for the greatest baseball town in the West.

And from "Where They Are: Gossip of Old-Time Western League Players Gone from Here," in the St. Paul [Minnesota] Globe (June 10, 1900):

In the Western association there are quite a number [of former Western League players]. ... But we are not able to find the names of Jack Pickett, Lou Camp, Pepper, Eddie Boyle, Mullane, Tim O'Rourke, Lefty Marr, Tacks Parrott and many others who have either quit or torn for the "Bush League." When you stop to think, they come and go pretty fast. Of the Western players who went to the big league this spring not much can be said.

And from "Prairie Baseball: Where the Game Is Played to Enthusiastic Galleries: Great North Dakota League," in the Minneapolis [Minnesota] Journal (August 19, 1901):

To see a typical "bush" league, Minneapolis fans should go out to North Dakota, out on the wide, endless prairies, where trains are sometimes seen three times a week, where a town of 2,000 is a metropolis, and where managers fall dead when they see 200 "paid admissions" in the baseball park.For all that, the North Dakota league will furnish some of the more pretentious teams with some fast players and there is more than one embryo National leaguer out on the prairies of the flickertail state.

...

The ladies [who attend games] are fully as enthusiastic as the men. They understand all the intricacies of the great sport and "root" loyally and heartily. They come to the games decked out in the club colors, with horns and other instruments for manufacturing noise, and the small boys often bring tin cans full of stones, a device for "rattling" the opposing pitcher.

Four additional articles in the Minneapolis Journal or the St. Paul Globe mention "bush league" baseball through the end of 1901; and by 1902, a new minor league seems to have acquired the nickname "the Bush league." From "Sioux City's Decision: Its Club Will Enter the New North Tri-State League," in the Minneapolis [Minnesota] Journal (February 20, 1902):

Sioux City, Iowa, Feb. 20.—W. E. Lockhart, C. E. Hughson, Henry Metz and D. E. Kirby, promoting baseball in Sioux City, at a meeting yesterday afternoon decided not to accept the proffer of a berth in the Western League. Instead, Sioux City will belong to the North Tri-State League, composed of Rock Rapids, Sheldon, LeMars, Iowa; Flandreau, Yankton and Sioux Falls, S. D., generally known as the "Bush League."

All of these early newspaper accounts come from cities (Kansas City, Minneapolis, and St. Paul) that hosted teams in the Western League—a step down from the "big league" (the National League) but considerably larger than even the largest of the Northern Tri-State cities. Although the 1899 reference to "the Bush league" as a place where Western League players might hope to receive relatively appealing contract offers seems utterly bogus, the league itself is treated as so remote that it seems rather exotic.

Subsequently, though, the league's environment comes into focus, with rudimentary field conditions and rowdy crowds trying to rattle opposing teams' players. The pejorative sense of "bush league" as amateurish or unprofessional surely flowed from the real or imagined conditions in small towns like those in the Northern Tri-State League around 1900.

On the basis of the Elephind newspaper database search results, it seems very likely to me that "bush league" as a term originated during the very late nineteenth century, in the U.S. Midwest, in a region bounded by Kansas and Missouri in the south and North Dakota and Minnesota in the north.

Solution 3:

I've been working with Paul Dickson to antedate earliest uses for a possible new edition of the Dickson Baseball Dictionary. The two earliest I've found for "bush league" are from January 1896:

“The baseball public has been very kind to me. But I could not get my price here, and, as I could not go to any club playing under the national agreement, I must go into a ‘bush’ league.” Ted Breitenstein, quoted in St. Louis Globe-Democrat, January 13, 1896, p9

“The Southern Illinois league consists of Belleville, Cairo, East St. Louis, Springfield, Edwardsville, Chester and Sportsman’s Park, St. Louis. It is a ‘bush’ league, outside the pale of the national agreement.” Rockford Daily Register-Gazette, January 15, 1896, p7