"Dost level best" actually means "to try one's hardest," so potential antonyms might include "half-hearted," "negligent," or "unmotivated." These words would describe someone who shows little interest or effort in a particular task, is easily distracted, or lacks the necessary motivation to achieve a goal. When someone does not "dost level best," they are more likely to cut corners, take shortcuts, or simply give up when faced with obstacles. In contrast, those who strive for excellence, pursue their goals with passion and determination, and work tirelessly to succeed are said to be "doing their dost level best.