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Adjective: thankless  thangk-lus
  1. Not likely to be rewarded
    "grading papers is a thankless task";
    - unappreciated, ungratifying
  2. Not feeling or showing gratitude
    "How sharper than a serpent's tooth it is / To have a thankless child!";
    - ungrateful, unthankful

See also: unappreciative, unrewarding