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]