The term is commonly used to describe left-handed pitchers. Yahoo Answers says there are reliable sources that the term was coined in Chicago where the stadium faced east and west with home plate on the west. Thus a left-handed pitcher threw from the south, and Finley Peter Dunne coined the term “Southpaw.”  Answerbag.com says it was common to build stadiums with home plate facing east so the late afternoon sun would not be in the hitter’s eyes.