The Collaborative International Dictionary of English v.0.48:

Undervalue \Un`der*val"ue\, v. t.
   1. To value, rate, or estimate below the real worth; to
      depreciate.
      [1913 Webster]

   2. To esteem lightly; to treat as of little worth; to hold in
      mean estimation; to despise.
      [1913 Webster]

            In comparison of it I undervalued all ensigns of
            authority.                            --Atterbury.
      [1913 Webster]

            I write not this with the least intention to
            undervalue the other parts of poetry. --Dryden.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Undervalue \Un`der*val"ue\, n.
   A low rate or price; a price less than the real worth;
   undervaluation. --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

undervalue
    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: esteem lightly
    3: lose in value; "The dollar depreciated again" [syn:
       depreciate, undervalue, devaluate, devalue] [ant:
       appreciate, apprise, apprize, revalue]