Antonyms for the word "under estimate" would be "overestimate" or "overrate." While underestimating someone or something means to undervalue their potential or abilities, overestimating means to overrate or overvalue them, giving them more credit than they deserve. When it comes to decision-making or judgement, overestimating can lead to unrealistic expectations or misplaced trust, while underestimating can result in missed opportunities or underutilized resources. Therefore, it is important to strike a balance between the two and make a fair and unbiased assessment of situations or individuals, without either underestimating or overestimating them.