Means "You are incompetent or unintelligent." Derives from a bizarre fusion of "you're retarded" and "You're a retard." It's intentionally unclear whether "tarded" is a noun or "a-tarded" is an adjective \.
1) A nickname for Avatar fans. Some fans have even been diagnoised with depression after seeing James Cameron's film, claiming that they want to live in a fictional world.
2) In 2006, people (7-16 year old girls) were 'Wild Cats!' following the High School Musical 'phenomenom' (?). In the first part of 2009, people were 'Twi-Hards' = Twilight(?), and in the second part, people were 'Gleeks' = Glee.