A combination of the words 'Jared' and 'beret'. It is a unique insult mainly done by those suffering with the 'tism and 'tardation, but may also be used as a natural response to insult someone for doing/saying something stupid.
Person 1: Look at this magic trick I can do! I can disappear until the table!
Person 2: Stop being such a Jeret!
Person 1: I came 2nd in that race because you clearly cheated!
Person 2: Ohhh of course, typical Jeret attitude.