-A person that finishes only 50 percent of something that they start.
-A lazy person
First mentioned by Pauly Shore in the 80's, but made popular by Larry the Cable Guy's self deprecating humor.
Also used in some of Jay Chandresekhar's Broken Lizard Industries movies
Jay Chandresekhar: "I dunno, he never finishes anything"
Larry the Cable Guy: "That's why they call me Mr. 50 Percent...giterdone!"