The Collaborative International Dictionary of English v.0.48:

Underestimate \Un`der*es"ti*mate\, v. t.
   To set too low a value on; to estimate below the truth.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Underestimate \Un`der*es"ti*mate\, n.
   The act of underestimating; too low an estimate.
   [1913 Webster]

WordNet (r) 3.0 (2006):

underestimate
    n 1: an estimation that is too low; an estimate that is less
         than the true or actual value [syn: underestimate,
         underestimation, underrating, underreckoning]
    v 1: assign too low a value to; "Don't underestimate the value
         of this heirloom-you may sell it at a good price" [syn:
         undervalue, underestimate] [ant: overestimate,
         overvalue]
    2: make a deliberately low estimate; "The construction company
       wanted the contract badly and lowballed" [syn: lowball,
       underestimate]
    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: underestimate,
       underrate] [ant: overestimate, overrate]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "underestimate":
   belittle, deprecate, depreciate, discount, disparage,
   make light of, make little of, make nothing of, minimize,
   miscalculate, misestimate, misjudge, misprize, sell short,
   set at naught, set little by, shrug off, think little of,
   think nothing of, underprize, underrate, underreckon, undervalue