John Dwight Chesbro (June 5, 1874 – November 6, 1931) was a Major League Baseball pitcher at the turn of the 20th century. He was the last major league pitcher to win 40 games or more in a single season until Ed Walsh did it in 1908. Chesbro was elected to the Baseball Hall of Fame in 1946.
Nicknamed "Happy Jack", Chesbro, a spitballer (spitballs were legal until 1920), broke into the majors in 1899 with the Pittsburgh Pirates. He had a 21-10 record with a 2.38 ERA in 1901 and a 28-6 record with a 2.17 ERA in 1902. His 28 wins led the National League.
In 1903 Chesbro moved to the newly formed New York Highlanders (soon to be New York Yankees) and pitched the franchise's first game. He finished the season with a 21-15 record.
In 1904, he had one of the finest years in the history of pitching, winning 41 games. He started 51 games and finished 48 while posting a 1.82 ERA, 239 strikeouts, and 454.7 innings. He nearly led the Highlanders to the 1904 American League pennant over the Boston Americans, but lost the last game of the season when one of his spitballs got away from him in the top of the ninth. Boston scored on the resulting wild pitch, and the Highlanders were shut out in the bottom frame.
His 41 wins are the most ever for a modern-era baseball season. In addition to his 51 starts, he also made 4 relief appearances, getting a decision in 96% of them, for a record of 41-12. Under current playing practices, where teams typically maintain a four- or five-man pitching rotation, his record is virtually unbreakable: the only pitcher since World War II to win 30 or more games in a season was Denny McLain who went 31-6 in 1968.