IPA: //ˌʌn.kənˈsiː.və.bəl//
KK: /ʌnˈkɒnsɪvəbəl/
Something that cannot be imagined or understood; too strange or unusual to be thought possible.
The idea of flying cars seemed unconceivable just a few decades ago.
Unconceivable → It is formed from "un-" (meaning not) and "conceivable" (from Latin "concipere", meaning to take in or grasp). The word "unconceivable" means something that cannot be grasped or understood.
Think of something that is 'not' ('un-') able to be 'grasped' or 'understood' ('conceivable'). This helps you remember that 'unconceivable' refers to something that is beyond comprehension.