Button down is commonly used to describe a type of collar on a dress shirt that is fastened to the shirt itself with buttons. However, when it comes to antonyms for button down, there are several terms that could be used. One possibility is "unbuttoned," which would suggest a casual, relaxed style with an open collar. Another option could be "loose" or "unstructured," emphasizing a lack of formality or strictness in clothing or behavior. Other antonyms might include "untucked," "untailored," or "informal," all suggesting a less refined or rigid appearance. Ultimately, the choice of antonym depends on the specific context and desired connotations.