“Their noble pitcher, Thomas Theodore Hinton, was knocked out of the box in the first inning, with his round house curves.” Evening Bulletin (Decatur, IL), August 6, 1896, p1
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1899. “The [Kid] Nichols curve is none of your roundhouse benders” (Buck Freeman, quoted in The Washington Post, Oct. 15; Peter Morris).