Someone who is so retarded to the point where the "re" is taken off the rest of the word.
Matt: Chris is such a tard. He is so caught up in his bullshit to the point that he forgot where his class was, and then yelled at the teacher when she told him it was the wrong class.
A derisive term meant to describe a mentally retarded person or more accurately, a person who acts stupidly. The evolution of the word comes as the word gets shorter, the greater the insult it conveys. Thus:
Mentally Retarded Person < Retarded < Retard < Tard.
Girl One: Mike is wearing his hoodie backwards again.
Girl Two: He’s such a tard!