"Not in a month of Sundays" is a popular idiom used to describe something that is highly unlikely or improbable. There are several synonyms that can be used to convey a similar meaning. One such phrase is "fat chance," which denotes that the odds of something happening or being successful are not in one's favor. "Next to impossible" is another synonym that can be used, indicating that something is exceedingly difficult to achieve or accomplish. "Virtually impossible" and "unachievable" are also commonly used phrases with similar meanings to "not in a month of Sundays." Using these alternatives can help you express your thoughts more effectively and creatively in conversations and writing.