from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Impossible to imagine; inconceivable: an unthinkable amount of money.
- adj. Contrary to what is plausible or probable: That this project would achieve ultimate success was unthinkable at the time.
- adj. Not to be thought of or considered; out of the question: Raising taxes was politically unthinkable.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. Incapable of being believed; incredible.
- adj. Inconceivable or unimaginable; extremely improbable in a way that goes against common sense.
from The Century Dictionary and Cyclopedia
- That cannot be made an object of thought; that cannot be thought; incogitable.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- adj. incapable of being conceived or considered
un- + thinkable (Wiktionary)