Me: Wow... because you said that dis you went from nobody loving you... to having a small squirl start to love you... oh wait... that squirl just got ran over by a car... your back to nobody loving you...
Faggot: I know you are but what am I?
Just a juvenile thing.. Used so often though (way back when) that til this day, I use the phrase in response to good compliments.. Most people my age, or close too, enjoy and laugh :)
Example: "You're cute" my response "I know you are but what am I" :)
Person 2: I think they rawk. iPhones for everyone!
Person 1: You're a poo brain.
Person 2: I know you are but what am I?