This is used as a comeback to any insult. It is meant to confuse the other person, giving you the last words Example: Chris: Donnie, you're a pilark. Donnie: I know you are but what about the table?
I KNOW YOU ARE BUT WHAT ABOUT
Meaning of I KNOW YOU ARE BUT WHAT ABOUT in English
Slang English vocab. Английский сленговый словарь. 2012