John Dwight Chesbro (June 5, 1874 – November 6, 1931) was a Major League Baseball (MLB) pitcher. Nicknamed "Happy Jack", Chesbro played for the Pittsburgh Pirates (1899–1902), the New York Highlanders (1903–1909), and the Boston Red Sox (1909). Chesbro finished his career with a win-loss record of 198-132, an earned run average of 2.68, and 1,265 strikeouts. His 41 wins during the 1904 season remains an American League record. Though some pitchers have won more games in some seasons prior to 1901, historians demarcating 1901 as the beginning of 'modern-era' major league baseball refer to and credit Jack Chesbro and his 1904 win-total as the modern era major league record and its holder. Some view Chesbro's 41 wins in a season as an unbreakable record.