Impeachable :: Impeachable (a.) That may be impeached; liable to impeachment; chargeable with a crime.
Unimpeachable :: Unimpeachable (a.) Not impeachable; not to be called in question; exempt from liability to accusation; free from stain, guilt, or fault; irreproachable; blameless; as, an unimpeachable reputation; unimpeachable testimony..