unthinkable

US /ʌnˈθɪŋ.kə.bəl/
UK /ʌnˈθɪŋ.kə.bəl/
1.

생각할 수 없는

so shocking that it cannot be imagined as possible:

:
Today, we see growing acceptance of gay marriage - something that would have been unthinkable a generation ago.