The phrase "wast obligation" refers to a duty or responsibility that is no longer needed or relevant. There are several synonyms for this term, including "obsolete obligation," "outdated duty," "void responsibility," and "irrelevant mandate." Each of these words can be used to describe a situation where an obligation that was once important or necessary has now become outdated or unnecessary. Whether due to changes in society, technology, or personal circumstances, obligations can sometimes become wasteful or obsolete. Using synonyms like these can help to convey the same idea in different ways, and allow for greater variety and nuance in your writing.