Button up is a popular phrasal verb that refers to fasten clothes by means of buttons. However, there are many other words that one can use instead of button up, such as fasten, zip, clip, snap, hook, buckle, and tie. All of these words are used to describe the act of securing clothing so that it doesn't come undone, and each one has a slightly different meaning or connotation. For example, zip implies a quick and easy closure, while tie suggests something more formal or dressy. Regardless of which word you choose, the important thing is to make sure your clothes are properly secured.