Strikeout King (1905)

“The Memphis boy struck out nineteen men, thereby equaling a mark hung up by Charlie Sweeney, a pitcher of the Providence club, in 1884, and companion of the famous Radbourne, an early strike-out king.” New Orleans Item, July 23, 1905, p6

Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1916. “Last year [Dave Danforth] was the undisputed strikeout king of the American Association” (Baseball Magazine, July; Peter Morris).