The word "impeccant" means faultless, perfect or flawless. Therefore, antonyms for "most impeccant" would be words that convey the opposite meaning. Some antonyms for this word could be imperfect, flawed, faulty, blemished, defective or tarnished. These words describe something that has errors, lacks quality or has negative traits. When describing a person, some antonyms for "most impeccant" could be dishonest, corrupt, immoral or unethical. It is important to note that the severity of the opposite meaning can vary, but the general idea is that something or someone is not faultless or perfect.