A very stupid way of saying "for real".
People who find this annoying phrase funny, or worse, actually use it in conversation, whether real or online, are complete and total retards.
It should also be noted that there is a positive correlation between how much you use this phrase and just how big of a retard you really are.
Person 1: "Hey, my grandma is a leprechaun!"
Person 2: "For cereal?"
Person 1: "No, yours is you retard."