The phrase "is above-board" means that something is legitimate, honest, and open. The antonyms of this phrase include words such as fraudulent, deceptive, and deceitful. When something is fraudulent, it means that it is a sham or a fake, and not legitimate. Deceptive refers to something that is misleading, and not truthful. Deceitful refers to something that is intentionally misleading or dishonest. Other antonyms of "above-board" include words such as illegal, dishonest, and underhanded. All these words describe actions or behavior that are not open, honest, and legitimate, and are, therefore, not above-board.