The antonyms for the phrase "doeth one's best" would be words such as "underperform," "slacker," "half-hearted," "sloppy," and "careless." These words all connote the opposite of putting forth one's utmost effort or attempting to achieve the highest possible outcome. A person who is not "doing their best" may be characterized as someone who is "half-assing it," "lazy," or "unmotivated." When it comes to achieving success or reaching goals, doing one's best is vital to ensure that one can be proud of their accomplishments and know that they have given their all to the task at hand.