Source : Webster's Revised Unabridged Dictionary (1913)

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

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

Source : WordNet®

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