Source : Webster's Revised Unabridged Dictionary (1913)

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

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

   2. Not obtaining or deserving thanks; unacceptable; as, a
      thankless task.

            To shepherd thankless, but by thieves that love the
            night allowed.                        --Chapman.
      -- {Thank"less*ly}, adv. -- {Thank"less*ness}, n.