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.