It's a phrase used to answer a stupid question or to counter a stupid opinion. Its produced by a statment created by Crash Holley (WWE star). He was protecting his hard core subject in a falls count anytime match. He was at a circus whenever a clown approached and believed to him, "i have seen on television. You have defended your subject regarding the street, in an alley and at a circus." Crash replied "a circus?". Simply then clown attacked him and tried to pin him. So their response to the puzzling statment had been "a circus?" It should be said in a loud, high picthed, questioning way.