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 ]