The Collaborative International Dictionary of English v.0.48:

lefty \lefty\ n.
   1. a person who uses the left hand with greater skill than
      the right; a left-handed person.

   Syn: lefthander, southpaw.
        [WordNet 1.5]

   2. a baseball pitcher who throws the ball with the left hand.

   Syn: left-handed pitcher, left-hander, southpaw.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

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