It means "sorry", used by people in gaming mostly and is closest associated with Nintendo Wii, so this would be used to say sorry for killing someone or messing up their game on a Wii game. Also see wiikend.
Person 1: Sorii dewd I had to snipe you.
Person 2: Ass.
Person 1: What!?¿
Person 2: Yeah, I'm sorii too.