The word "unfailing" means something that is dependable, reliable, and consistent. The antonyms for "unfailing" are words that describe something that is not reliable and inconsistent. These terms include terms like erratic, uncertain, untrustworthy, unreliable, and inconsistent. An alternative set of antonyms might include terms such as sporadic, fickle, fluctuating, variable, and volatile. These terms describe things that are not dependable, that are subject to change without warning, and that cannot be relied on to act or perform in a consistent manner. Whether we are describing a person, a process, or a set of circumstances, the antonyms for "unfailing" serve to underscore the fragility of human relations and the inherent uncertainties of life.