Antonyms for the word "over-estimating" include: underestimating, undervaluing, downplaying, diminishing, belittling, devaluing, and trivializing. Underestimating refers to the act of thinking that something is smaller, less important, or less valuable than it actually is. Undervaluing refers to thinking that the value of something is less than its true worth. Downplaying refers to minimizing or reducing the importance of something. Diminishing refers to making something appear smaller or less significant. Belittling is a form of ridicule that seeks to make someone or something seem insignificant or unimportant. Devaluing refers to thinking that something has lost its worth, value or importance. Trivializing refers to making something seem unimportant or irrelevant.