One-Hitter (1913)

Jimmy Dygert, who has been nursed along carefully until he felt ‘right,’ certainly showed marvelous form, and his one-hitter was no freak, accident, or gift, but the real thing.The Shreveport Times, April 4, 1913, p3

Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1918. “[Lefty Gregg] pitched a one-hitter against St. Louis” (Lincoln [Neb.] Evening State Journal, June 4, p. 9; Benjamin Zimmer).