I know you are but what am I

phrase3

1. •
   (rhetoric, humorous) Used to assert that an insult made by the party to whom the phrase is directed is actually true of that party, and not of the person using the phrase. Usually considered to be a playground taunt.

   • I know you are but what am I? Jeannie would say out loud, whenever I mouthed a name at her. — 1984
2. •
   (humorous, rare) Used to direct back a statement as a compliment.

   • > Vince, you're cute! O.o I know you are but what am I? — 1999
3. •
   (Internet, rhetoric) Used to mock a retort in a dispute that tries to reverse an insult on its original speaker.