Fraud (ghish) is concealing from people some fault, blemish, or harm, either of a religious or worldly nature. Others have said that fraud is making something useless or defective seem useful and beneficial, or making something bad appear to be good. One of the most widely transmitted hadith in the Islamic tradition is the Prophet's saying, "Whoever defrauds us is not one of us." Sacred Law forbids selling something without pointing out its defects. If the seller conceals defects or fails to disclose them intentionally, this is fraud, whether its victim is a Muslims or not.
The Sophists of ancient Greece loved and practically worshipped rhetoric. They were the first historical relativists, in that they held the theory that right and wrong do not exist in an objective and transcending sense. Whoever makes the most skilled and persuasive argument is right. The Sophists believed that the most important thing is to be convincing, whether one is telling the truth or lying, whether one is defending corruption or upholding justice. This is fraud of the tongue.
Rhetoric was also an art form in Islamic literacy and oratorical history. But to the Muslim, rhetoric was the art of embellishing the truth and presenting it persuasively.