A phrase used by teenage girls to refer to any person that they don't yet hate with the fiery passion of 1000 burning suns.
Male person: Who's your best friend?
Teenage girl: Sally.
Male person: Wait, wasn't Jared your best friend last week?
Teenage girl: He's my best friend too, but Sally's my BEST friend.
Male person: So what about Haley?
Teenage girl: Oh, she's my best BEST friend.
Male person: ... Okay, what about Susan?
Teenage Girl: Oh, HER. She WAS my best friend until she wore the same dress as me to Chris's party last week. Susan! Wearing the same dress as me to my best friend's party! I mean really!?
Male person: *walks away confused and with a drop of at least 5 points in his IQ level*