An slang term that began in the late 70's. The word sike is used to negate what ever statement might have come before it. It basically means, "just kidding" or "yea, right". Usually, a pause is given between the original statement and the "sike" as to see if the original comment caused a reaction, before people realize that the person was just kidding. It is used in exactly the same way "Not!" was used, when coined by the Wayne's World movies.
George Bush is the greatest president ever... Sike!