Buttoning up is often associated with completing a task or preparing for something. However, there are many antonyms that describe actions that are the opposite of buttoning up. For instance, you could consider "letting loose" as an antonym of buttoning up, which refers to relaxing and being carefree. Another antonym is "unfastening," which means to undo a closure or release something. "Loosening" is another antonym, which refers to making something less tight or tense. Finally, "letting go" describes the act of releasing control or allowing something to happen naturally, in contrast to the intentional action of buttoning up.