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unthinkable

[uhn-thing-kuh-buh l]
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adjective
  1. inconceivable; unimaginable: the unthinkable size of the universe.
  2. not to be considered; out of the question: Such a suggestion is unthinkable.
noun
  1. something that cannot be conceived or imagined, as something too unusual, vague, or disagreeable: Today's unthinkables are tomorrow's realities.

Origin of unthinkable

late Middle English word dating back to 1400–50; see origin at un-1, thinkable
Related formsun·think·a·bil·i·ty, un·think·a·ble·ness, nounun·think·a·bly, adverb
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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British Dictionary definitions for unthinkable

unthinkable

adjective
  1. not to be contemplated; out of the question
  2. unimaginable; inconceivable
  3. unreasonable; improbable
Derived Formsunthinkability or unthinkableness, noununthinkably, adverb
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for unthinkable

adj.

early 15c., "too large to be conceived, unimaginable," from un- (1) "not" + thinkable (see think). Meaning "incapable of being framed by thought" is recorded from mid-15c.

Online Etymology Dictionary, © 2010 Douglas Harper