The antonyms for the phrase "do one's best" are varied, depending on the context. Some of the most common antonyms include "half-heartedly," "slack off," "give up," and "phone it in." These antonyms suggest a lack of effort or commitment to the task at hand. Other possible antonyms might include "do it poorly," "mess up," or "fall short." Ultimately, the opposite of doing one's best is failing to put forth one's full effort or failing to achieve the desired outcome. As such, the key to success in any endeavor is to remain focused, committed, and motivated to achieve one's goals.