from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Not feeling or showing gratitude; ungrateful.
- adj. Not likely to be appreciated: a thankless job.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. not appreciated or rewarded
- adj. ungrateful or unappreciative
from the GNU version of the Collaborative International Dictionary of English
- adj. Not acknowledging favors; not expressing thankfulness; unthankful; ungrateful.
- adj. Not obtaining or deserving thanks; unacceptable.
from The Century Dictionary and Cyclopedia
- Unthankful; ungrateful; not acknowledging kindness or benefits.
- Not deserving thanks, or not likely to be rewarded with thanks: as, a thankless task.
- Synonyms See grateful.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- adj. not feeling or showing gratitude
- adj. not likely to be rewarded
From thank + -less. (Wiktionary)