Source : Webster's Revised Unabridged Dictionary (1913)

Underestimate \Un`der*es"ti*mate\, v. t.
   To set to? low a value on; to estimate below the truth.

Underestimate \Un`der*es"ti*mate\, n.
   The act of underestimating; too low an estimate.

Source : WordNet®

underestimate
     n : an estimation that is too low; an estimate that is less than
         the true or actual value [syn: {underestimation}, {underrating},
          {underreckoning}]
     v 1: assign too low a value to; "Don't underestimate the value of
          this heirlooom-you may sell it at a good price" [syn: {undervalue}]
          [ant: {overvalue}, {overvalue}]
     2: make a deliberately low estimate; "The construction company
        wanted the contract badly and lowballed" [syn: {lowball}]
     3: make too low an estimate of; "he underestimated the work
        that went into the renovation"; "Don't underestimate the
        danger of such a raft trip on this river" [syn: {underrate}]
        [ant: {overestimate}]