Inconceivable or unimaginable; extremely improbable in a way that goes against common sense.
Near-synonyms: inconceivable, imponderable
Nothing is unthinkable, nothing impossible to the balanced person, provided it comes out of the needs of life and is dedicated to life's further development - Lewis Mumford
3 more examples
With 16 minutes left on the clock and the tension climbing through the roof, Trinh-Duc tried his luck with a penalty from just inside halfway only to push it wide, but the unthinkable now seemed a real possibility.2011 October 23, Tom Fordyce, “2011 Rugby World Cup final: New Zealand 8-7 France”, in BBC Sport, archived from the original on 27 Nov 2021:
The ruling also comes amid calls from an opposition party for a review of the royal insult law, which would have been unthinkable just a few years ago.2021 November 10, Panarat Thepgumpanat, Chayut Setboonsarng, “Thai court rules students' royal reform call sought to overthrow monarchy”, in Reuters, Reuters, archived from the original on 26 Mar 2022, retrieved 10 Nov 2021:
It reopened as St Pancras Renaissance Hotel London in the early part of this century, and it now seens unthinkable to demolish such a structure.2023 August 23, Dr Joseph Brennan, “Check out Britain's famous station hotels: St Pancras”, in RAIL, number 990, page 61:
3
Extremely improper; extremely at odds with norms of expected behavior.
I want you to know that P.A.C. will pay for a child psychiatrist to come in and testify on behalf of the child so that the homosexual father will not be able to have that little girl overnight and weekends to expose her to what is unthinkable.1980 December 20, Von Carswell, quotee, “Still At It”, in Gay Community News, volume 8, number 22, page 2:
unthinkable
noun
Etymology From un- + think + -able.
1
Something, or various things collectively, that one dare not think about, dare not do, or dare not think about doing.