A southpaw is any left-handed person, but in particular a left-handed pitcher in baseball.
Niese, a southpaw who has allowed just five homers in 124 1/3 innings at Double A Binghamton, has a deceptive delivery and gets very good movement on his average fastball.
— Joe Sheehan, Sports Illustrated, 4 Aug. 2008
A specious theory regarding southpaw ties its origin to the layout of baseball parks. To keep the afternoon sun out of batters’ eyes, diamonds were supposedly built so that the pitcher faced west—and his arm, therefore, would be pointed south.
But early use of the compound word south paw—referring to the left hand—doesn’t refer at all to baseball. In fact, the earliest citation from the Oxford English Dictionary comes from The Tickler, a short-lived humor magazine based out of London, where the sport did not have much of a following in the early 19th century.