1. a person of subnormal intelligence
2. Used as a disparaging term for a mentally retarded person.
3. A person considered to be foolish or socially inept
"Why are you acting like such an R-Tard"
"You will never get your word in a dictionary, bullshit R-Tards"
Typically used to reference people like the guy from the other entry randomly downplaying Republicans in a completely inappropriate setting - they think it's cute, but really, it's just rtarded.