Antonyms for the phrase "does one's best" can include terms like "half-hearted," "apathetic," "careless," "neglectful," and "irresponsible." These words describe an attitude of not putting effort into something or not fulfilling one's responsibilities. If someone is not doing their best, they may be cutting corners, taking shortcuts, or simply not giving their full attention to a task. This can lead to poor outcomes, mistakes, or even failure. It is important to strive to always do one's best, in order to achieve success, gain respect and confidence, and feel proud of one's accomplishments.
