Entry 2 senses · 2 variants Webster, 1913 Underestimate /ŭn'-dẽr-ĕs'-təm-ātˌ/ · Un·der·es·ti·mate · IPA /ʌndɚˈɛs.tɪ.meɪt/ v. t. n. 01 v. t. To set too low a value on; to estimate below the truth. 1. To set too low a value on; to estimate below the truth. 02 n. The act of underestimating; too low an estimate. 1. The act of underestimating; too low an estimate.