Originated in the ghetto streets of High Wycombe, England. Upon being insulted with the usual derogotory profanities, the person would then reply with this phrase. Making the person whol insulted you, think about what indeed are they. Realising they are nothing, this insult usually wins....
Colin: "you know, you really are a cock!"
Colin: "yeh, and your gay!"
Alger: "really? well tehn what are you?"