Similar to the common word "retard," only a "retard" is someone born with a disability who cannot help it. A "tard" is someone that is not innately mentally disabled. On the contrary, a tard is so damnably stupid and such a bellend, that they appear to be retarded to the untrained eye.
Someone whos is lacking all manners of intelligance. Someone whos is going out with a kid named David.
And insult used by awsome people with great observing skills to make dumb people want to kill themselves
"Jessica, you tard"
"Jessica shut-up you're a tard"
"Jessica you dumbass tard, stop arguing with James, hes always right"