IPA: /ɪnˈsʌpə(r)təbl/
KK: /ɪnˈsʌpərtəbl/
Something that cannot be justified or tolerated; it is too difficult or painful to bear.
The insupportable heat made it impossible to work outside.
Insupportable is formed from "in-" (meaning not) and "supportare" (from Latin, meaning to carry or support) and the suffix "-able" (meaning capable of). The word describes something that is not capable of being supported or endured.
Think of 'not' ('in-') being able to be 'carried' ('supportare') — that's why insupportable means something that cannot be endured.