Button-down refers to a type of dress shirt with buttons securing the collar points to the shirt's front. In this context, an antonym for button-down could be a collarless shirt or a shirt with a loose or open collar. Other antonyms of button-down could be casual, relaxed or informal. Button-down is also used to describe a personality or style that is conservative, conventional, and conformist. Its antonyms in this context could be radical, unconventional or daring. Therefore, the antonyms for button-down can be used to describe both the physical characteristics of a shirt and a person's personality or approach to life.