The expression "nails it" is used to indicate when someone has successfully achieved something or completed a task with excellence. Its antonymous expression would be "misses the mark," which denotes the opposite of nailing it, such as failing to achieve something, performing below expectations, or making a mistake. Other antonyms for "nails it" might include underperforming, struggling, messing up, or falling short. These terms imply a lack of competence, skills, or dedication. Therefore, when we use "nails it" or its antonyms, we are referring to the level of proficiency and success of someone's efforts or actions.