The phrase "am one's back" does not have any antonyms as it is not a commonly used phrase in English. It is possible that the intended phrase is "have one's back," which means to support or be loyal to someone. In this case, the antonyms would be "betray" or "abandon." However, if the intended meaning is "am physically located behind someone," some antonyms would be "in front," "ahead," or "before." It is important to clarify the intended meaning of a phrase to accurately determine its antonyms.