A "southpaw" is a left-handed pitcher.
For decades, the southpaw origin story was a logical one: In the days before lighting systems made night games possible, most ballparks were oriented so that the batter would be looking east out to the mound in order to avoid having to stare into the glare of the afternoon sun. So, with pitchers facing west when they stared into home plate, the arm of a left-handed hurler would be to the south side of the diamond.
However, the earliest mentions of the term in baseball refer to position players, not pitchers. According to MLB official historian John Thorn, baseball has a totally different sport to thank: boxing, in which hitting someone with a left hand came to be known as a punch using the "south paw."