An axe to grind refers to a personal interest or motivation that drives a person's actions or opinions. There are several synonyms for the phrase that depict similar ideas, such as hidden agenda, ulterior motive, personal vendetta, bone to pick, and chip on the shoulder. Each of these phrases implies a deeper, often personal reason behind a person's actions or opinions, suggesting a level of bias or self-interest. Knowing these synonyms can help to understand the reasoning behind one's actions and opinions and can aid in effective communication and addressing any underlying issues.