Buttoning down is a term used to describe the act of securing or tightening something. When it comes to antonyms, there are various opposite meanings for this word, depending on the context. One antonym for buttoning down could be "loosening up," which means to make something less restrictive or to relax. Another antonym could be "letting go" or "releasing," which refers to the act of setting something free or relinquishing control. Additionally, "opening up" or "unfastening" could be used as antonyms for buttoning down. These antonyms imply the act of making something less constricting, letting go of limiting beliefs, or releasing oneself from constraints.