IPA: //ʌnˈθɪŋkəbli//
KK: /ʌnˈθɪŋkəbli/
In a way that is impossible to imagine or comprehend; beyond what is reasonable or acceptable.
The situation escalated unthinkably, leaving everyone in shock.
Unthinkably → It is formed from "un-" (meaning not) and "think" (from Old English "þencan", meaning to conceive in the mind) and "-ably" (meaning in a manner that can be). The word "unthinkably" means in a manner that cannot be conceived or imagined.
Think of 'not' ('un-') being combined with 'conceiving in the mind' ('think') and 'in a manner that can be' ('-ably'). This helps you remember that 'unthinkably' refers to something that is beyond imagination.