Tone up is a phrasal verb that means to make one's body or muscles firmer or stronger. It is often used in the context of exercise and fitness. There are several synonyms for tone up that can be used interchangeably to express the same meaning. Some of these synonyms include firm up, strengthen, beef up, build up, bulk up, shape up, and trim down. These synonyms can be used in different contexts and situations. For example, firm up can be used to express the idea of making something more stable or secure, while build up can be used to suggest the idea of creating or developing something over time. Overall, these synonyms offer a variety of ways to express the concept of toning up.