(informal American) a person who favors their left hand, someone who is left-handed;
e.g. Strangely, he is a lefty when playing tennis but favors his right hand for everything else.
(noun)
(informal) a person who favors the principles and beliefs of the political left, characterized by liberal views in favor of revolution and reform of social, economic, and political systems;
e.g. She feared that her being the only lefty at the event would lead to some disagreements.