The antonyms for the phrase "cool heels" could be hot, feverish, agitated or frenzied. These words convey a more intense and anxious meaning, opposite to the relaxed persona of being "cool". Other antonyms could be unremarkable, dull, drab, bland or insipid, which suggests a lack of energy or interesting qualities. These words carry the opposite meaning of "cool" which conveys something different, fresh and unique. While "cool" could mean calm and collected, the antonyms mentioned here describe the opposite of that, conveying a sense of urgency, anxiety or commonness.