The antonyms for the phrase "giving it one best shot" include half-hearted effort, lack of determination, and disinterest. When someone is not fully committed to the task at hand, they tend to put in a lackadaisical performance, which leads to subpar results. The lack of enthusiasm is what makes this an antonym; instead of giving it their all, they are only giving a fraction of their effort. This can be detrimental in many areas of life, such as school, work, and personal relationships. It is essential to put in your best effort every time, regardless of the outcome, to ensure success and satisfaction with yourself.