Mispelling of the word "retard". People suffering from insomnia dyslexia may accidently spell retard in this way, and when used as an insult, the word backfires in your face and only proves that you are the retard.
Man1- What's 2+2?
Man2- 3.
Man1- No, it's 4. You're a retetard.
Man2- I was being sarcastic and you're the retard because you can't spell.
Somewhat similar to the concept of a frenemy, a "relatard" is someone who is retarded (not in the literal sense) that you only associate with because they're related to you by blood or marriage.