Un-impeachable means something that is beyond reproach or doubt. Therefore, antonyms for this word will include terms that describe something questionable, doubtful or impeachable. Some antonyms for un-impeachable include questionable, doubtful, untrustworthy, unreliable, suspicious, suspect, impeachable, and disreputable. These words describe something or someone that cannot be trusted and may have some imperfection or fault in their character. Therefore, when describing something that is not un-impeachable, it may be important to use such antonyms to illustrate the level of doubt or questioning on the subject.