The Collaborative International Dictionary of English v.0.48:

lefthander \left`hand"er\, left-hander \left`-hand"er\n.
   a person who uses the left hand with greater skill than the
   right; a left-handed[1] person.

   Syn: lefty, southpaw.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

left-hander
    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]