Unthinkable means too shocking to be considered at all. Simply unthinkable, the unthinkable happened, an unthinkable choice.
It is about refusal rather than difficulty: something unthinkable is something nobody is willing to think about. The noun use, thinking the unthinkable, is a standing phrase for facing exactly what everybody avoids.
1adjectiveunthinkableToo shocking to be considered at all.
“It was unthinkable that the champion would lose to an unknown teenager.”
unthinkable is a quality.
Middle English
thinken
English
think
English, today
unthinkable
From un- + think + -able. think comes from Middle English thinken. From Middle English thinken, thynken, thenken, thenchen, from Old English þenċan, from Proto-West Germanic þankijan, from Proto-Germanic þankijaną (“to think”), from Proto-Indo-European teng- (“to think, feel, know”). Cognate with Scots think, thynk (“to think”), frr teenk, taanke, tanke, tånke (“to think”), stq toanke (“to think”), West Frisian tinke (“to think”), Dutch denken, dinken (“to think”), Afrikaans dink (“to think”), Low German denken, dinken (“to think”), German denken (“to think”), Danish tænke (“to think”), Swedish tänka (“to think”), Norwegian Bokmål tenke (“to think”), Norwegian Nynorsk tenkja (“to think”), Icelandic þekkja (“to know, recognise, identify, perceive”), Gothic þagkjan (“to think”), Latin tongeō (“know”).
Etymology from Wiktionary, CC BY-SA