The Collaborative International Dictionary of English v.0.48:

Thankless \Thank"less\, a.
   1. Not acknowledging favors; not expressing thankfulness;
      unthankful; ungrateful.
      [1913 Webster]

            That she may feel
            How sharper than a serpent's tooth it is
            To have a thankless child!            --Shak.
      [1913 Webster]

   2. Not obtaining or deserving thanks; unacceptable; as, a
      thankless task.
      [1913 Webster]

            To shepherd thankless, but by thieves that love the
            night allowed.                        --Chapman.
      [1913 Webster] -- Thank"less*ly, adv. --
      Thank"less*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

thankless
    adj 1: not feeling or showing gratitude; "ungrateful heirs";
           "How sharper than a serpent's tooth it is / To have a
           thankless child!"- Shakespeare [syn: ungrateful,
           thankless, unthankful] [ant: grateful, thankful]
    2: not likely to be rewarded; "grading papers is a thankless
       task" [syn: thankless, unappreciated, ungratifying]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "thankless":
   bitter, bootless, careless, despised, disagreeable, dislikable,
   disliked, displeasing, distasteful, fruitless, futile, heedless,
   lowly, miserable, misprized, misunderstood, out of favor,
   profitless, self-centered, sour, thoughtless, ugly, unacknowledged,
   unalluring, unappealing, unappetizing, unappreciated,
   unappreciative, unattractive, uncared-for, undelectable,
   undelicious, undesirable, unengaging, unenjoyable, ungrateful,
   uninviting, unlamented, unlikable, unmindful, unmissed, unmourned,
   unpalatable, unpleasant, unpleasing, unpopular, unprofitable,
   unregretted, unrequited, unrewarding, unsavory, unsung, untasteful,
   unthankful, unvalued, unwelcome, useless, vain, wretched