Jack in is a phrasal verb that means to quit or abandon something. It can be used in various contexts, such as work, sports, or even relationships. There are several synonyms for the phrase "jack in," and some of them include give up, resign, quit, withdraw, step down, bow out, and throw in the towel. These synonyms can be used interchangeably, depending on the situation or context. For instance, someone can "jack in" their job or "throw in the towel" if they find it too difficult to continue. Overall, these synonyms give us more options to express the act of quitting or leaving something behind.