The third born child from a particular family. Usually used as a derogatory slur, the name "Third" implies that the individual is an accident or unwanted because of overpopulation.
The term originated from the book "Ender's Game". The setting takes place in the future and because of Earth's overpopulation, all families are only allowed a maximum of 2 children. However due to Earth's need for a super commander of all space forces, the Wiggin's are allowed to have a "third".
Child 1: Don't talk to him. He's a third.
Child 2: Yeah! Third! Third!