Wordnet 3.0
ADJECTIVE (1)
1. 
 not feeling or showing gratitude; 
- Example: "ungrateful heirs"- Example: "How sharper than a serpent's tooth it is / To have a thankless child!"- Shakespeare[syn: ungrateful, thankless, unthankful]
The Collaborative International Dictionary of English v.0.48:
Unthankful \Unthankful\
   See thankful.
WordNet (r) 3.0 (2006):
unthankful
    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]