The Collaborative International Dictionary of English v.0.48:

Southpaw \South"paw`\, a. (Baseball)
   Using the left hand in pitching; said of a pitcher. [Cant]
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Southpaw \South"paw`\, n.
   A pitcher who pitches with the left hand. [Cant]
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

southpaw
    n 1: a baseball pitcher who throws the ball with the left hand
         [syn: left-handed pitcher, left-hander, left hander,
         lefthander, lefty, southpaw]
    2: a person who uses the left hand with greater skill than the
       right; "their pitcher was a southpaw" [syn: left-hander,
       lefty, southpaw]