A phrase common among youngsters and especially young teenage girls, who often push their boyfriends to say it too. This phrase is used by people who want to sound like a 'gangsta' or 'rapper' and want to sound cool.
Don't use it.
Usually people say "hey that phrase is mine" when NO, really they heard it off someone else. You did not make this phrase up yourself so shut up.
Girl 1: "ahhhh Lisa that chicken mayo was soo tasty"
Lisa: "haha, yeah boii"
Bystander: "urgh, why does she say that, she's just trying to be cool"
Bystander 2: "yeah, what a loser"