Belt in is a phrasal verb that means confining or restraining someone or something. There are a few different synonyms for this phrase that can be used interchangeably in different contexts. One option is to use the term "rein in," which implies controlling or holding back something that is typically out of control. Another option is "constrain," which suggests limiting the actions or movements of someone or something. "Bind," "restrict," and "curb" could also be used as synonyms for "belt in," depending on the context in which it is used. Overall, these words all have a similar meaning to "belt in" and can be used when you need to convey a sense of confinement or control.