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]