un·der·es·ti·mate 
(ŭn′dər-ĕstə-māt′)
(ŭn′dər-ĕstə-māt′)tr.v. un·der·es·ti·mat·ed, un·der·es·ti·mat·ing, un·der·es·ti·mates
1. To make too low an estimate of the quantity, degree, or worth of: Don't underestimate the difficulties involved in the project.
2. To consider (someone) to be less capable or effective than is actually the case: underestimated his rivals and was outmaneuvered.
n. (-ĕstə-mĭt)
An estimate that is or proves to be too low.
un′der·es′ti·mation n.