Tard is a derogatory term which is an obvious shortening of the word "retard", which is, in turn a shoretning of "retarded".

The word retarded is an adjective which, when aplied to a human, usually describes a state in which their mental development is unusually slow or has halted as an early stage.

Tard, however, has come to mean a person that is stupid. However it tends to go farther than that. A Tard is a person who is actively stupid. A person who spits in the face of reason. A person who willfully refuses to learn from their mistakes.

See Also: I am militantly unaware of my environment