"Belt up" is a phrasal verb that can be used to mean "to fasten your seat belt" or "to be quiet". When it comes to finding synonyms for this phrase, different meanings will align with different options. For example, some alternative phrases to indicate the act of fastening a seat belt are "buckle up" or "secure your seat belt". Likewise, when asking someone to be quiet, you might say "hush" or "shut up". Another option for this latter meaning is "zip it". Regardless of the contexts in which you use this phrase, knowing synonyms can help you avoid repetition and infuse your communication with more variety.