Jack Chesbro (June 5, 1874 - November 6, 1931) was a Major League Baseball pitcher at the turn of the 20th century. He was nicknamed "Happy Jack".
Chesbro, a spitballer (spitballs were legal until 1920), broke into the majors in 1899 with the Pittsburgh Pirates. He pitched for the Pirates until 1902 and in his final year went an astonishing 28-6 with a 2.17 ERA.
In 1903 Chesbro moved to the newly formed New York Highlanders (soon to be New York Yankees) and pitched the franchise's first game. In 1904 he had one of the finest years in the history of pitching, starting 51 games and finishing 48 while posting a 1.82 ERA, struck out 239 batters, and 41 wins over 454.7 innings pitched. On the last day of the season, though, in a game against Boston, Chesbro threw a wild pitch in the top of the 9th inning, allowing the winning run to score from third base and causing the Highlanders to lose the pennant to Boston.
Chesbro retired in 1909, having accumulated a 198-132 career record and been part of two pennant-winning teams (in 1901 and 1902).
Chesbro was inducted into the Baseball Hall of Fame in 1946.
His 1904 record for games won in a season (41 wins) has stood for over a century, which is one of the oldest major records in baseball, or in any other sport. Under current playing practices, his record is unbreakable. Chesbro started 51 games that season (plus 4 relief appearances) and pitched 48 complete games, for a record of 41-12. Today, it is uncommon for a pitcher to start even 35 games in a season and complete games are a rarety.