Antonyms for the phrase "dressed to teeth" could include "underdressed," "unkempt," or "sloppy." When someone is "dressed to teeth," they are typically dressed in a specific, polished manner. However, the opposite of that would be to dress down or not put any effort into their appearance at all. Someone who is underdressed may appear too casual or not appropriately dressed for a particular occasion. Alternatively, someone showing signs of being unkempt or sloppy may appear scruffy, untidy, or disheveled. As such, these antonyms shine a light on the importance of putting effort into presenting oneself well in different contexts.