It's supposed to be a person obsessed in Hetalia, but there's another meaning to it. The "Heta" in "Hetalia" comes from the word "Hetare" in Japanese, which means "useless".
So the meanings are:
1. Useless retard, "retard" because of the "tard" in the word
Girl One: Hey! Are you going out with Derp anymore?
Girl Two: No, he's such a hetard
Girl One: A what?
Girl Two: A useless retard
Girl One: Oh...
~~♥
Girl One: I bought Hetalia plushies and posters and dvd's and etc etc etc...
Girl Two: Wow, you've become a Hetard
Girl One: No! I've become one withMother Russia!
Girl Two: o_o...
Did you hear, those dumb feminists, instead on focusing on actually oppressed women in the Middle East, made a new word again, I think it was Hetard or something, because every man that doesn't think that air conditioning is sexist is retarded, apparently lol.