1. Impossible to imagine; inconceivable: "The inert gases have been coaxed into combination, and I have seen a fluoride of xenon, unthinkable in the 1940s" (Oliver Sacks).
2. Not worth considering because of being socially unacceptable, too difficult, or too risky: Raising taxes was politically unthinkable.
3. Abhorrent; monstrous: unthinkable brutality.
un·think′a·bili·ty, un·thinka·ble·ness n.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.