The Collaborative International Dictionary of English v.0.48:

Underrate \Un`der*rate"\, v. t.
   To rate too low; to rate below the value; to undervalue.
   --Burke.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Underrate \Un"der*rate`\, n.
   A price less than the value; as, to sell a thing at an
   underrate. --Cowley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

underrate
    v 1: 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:

27 Moby Thesaurus words for "underrate":
   belittle, decry, deprecate, depreciate, devaluate, devalue,
   disparage, lower, make light of, make little of, make nothing of,
   mark down, minimize, misestimate, misprize, sell short,
   set at naught, set little by, shrug off, think little of,
   think nothing of, underestimate, underprize, underreckon,
   undervalue, write down, write off