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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·bil′i·ty, un·think′a·ble·ness n.
un·think′a·bly adv.
ThesaurusAntonymsRelated WordsSynonymsLegend:
Adv.1.unthinkably - to an unimaginable extentunthinkably - to an unimaginable extent    
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References in classic literature ?
Now that you are about to die the most unthinkably horrid death that it is given a white man to die--let this word of the plight of your wife add to the torments that you must suffer before the last savage spear-thrust releases you from your torture.
Above him, falling upon him like a bolt from the blue, was a winged hawk unthinkably vaster than the one he had encountered.
and similar unthinkably better and wiser and more virile congregating-places, that will receive the men who now go to saloons to find themselves and one another.
Ten thousand unthinkably atrocious deaths could not atone for the affront that you have put upon me.
One--twice--thrice that terrifying cry rang out across the teeming solitude of that unspeakably quick, yet unthinkably dead, world.
You know the wonder of a woman's arm--the fore arm, I mean; the sweet fading away from rounded biceps and hint of muscle, down through small elbow and firm soft swell to the wrist, small, unthinkably small and round and strong.
Such a forest, such a jungle might have stood there in the beginning of that unthinkably far-gone age before God peopled the world with life, when there were no sounds because there were no ears to hear.
Her home was unthinkably filthy; you could not enter by the front door at all, owing to the mattresses, and when you tried to go up the backstairs you found that she had walled up most of the porch with old boards to make a place to keep her chickens.
Yet now, resolutely, as only a man can do who is capable of martyring himself for the cause of science, he proceeded to violate all the fineness and delicacy of his nature by making love to the unthinkably disgusting bushwoman.
But mystery surrounds how the dog got into the bin in the first place - whether it jumped in or, unthinkably, was dumped there.
James Cameron, almost unthinkably, delivered a superior sequel, Aliens, but sadly it was all downhill from there.
And yet the stasis has been going on so long that, almost unthinkably, there's a serious risk he leaves the club in a worse position than they were when he took over.