IPA: //ʌnˈθɪŋkəˌbɪlɪti//
KK: /ʌnˈθɪŋkəˌbɪlɪti/
The quality or state of being impossible to think about or imagine.
The unthinkability of such a disaster left everyone in shock.
Unthinkability → It is formed from "un-" (meaning not) and "think" (from Old English "þencan", meaning to conceive in the mind) and "-ability" (from Latin "abilitas", meaning capacity or quality). The word "unthinkability" refers to the quality of being unable to conceive or imagine something.
Think of 'not' ('un-') being able to 'conceive in the mind' ('think') — that's why unthinkability means something that cannot be imagined.