I know you are but what am I

Meanings

1. 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.
2. Used to mock a retort in a dispute that tries to reverse an insult on its original speaker.
3. Used to direct back a statement as a compliment.

Etymology

Uncertain, but dates back at least to the June 15th, 1957 issue of The New Yorker (Vol. 33 No. 17, page 23). In "The Talk of the Town", which was a collaborative effort by the magazine's staff writers: "INTERROGATIVE WISP FOUND IN GUTTER OF MACDOUGAL STREET: 'I know you are but what am I?'" The phrase is often attributed to Pee-Wee Herman.