Undissembling means being honest and straightforward. Antonyms for undissembling would be deceitful, dishonest, insincere, shady, disingenuous, and fraudulent. To say someone is undissembling means they are being truthful and sincere in their interactions. Someone who is the opposite of this would be someone who is deceitful or shady, someone who is not truthful in their interactions. Being undissembling implies a sense of trust and reliability, making one feel confident in their interactions with the person. Using the opposite antonyms creates a sense of dishonesty and lack of trust, giving off a negative impression.