Slacken off is a term that means to become less tense or to loosen up. There are many synonyms for the word "slacken off" that can be used, depending on the context in which it is used. Some synonyms include relax, ease up, decrease intensity, reduce speed, and diminish pressure. Other synonyms for slacken off include let up, slow down, ease off, and lighten up. These words convey the same meaning as slacken off and can be used interchangeably depending on the context. In summary, there are numerous synonyms for the verb slacken off, which can be used to communicate the same idea in different ways.