IPA: /ʌnˈɛndjʊərəblɪ/
KK: /ʌnˈɛndʒərəbli/
In a way that cannot be tolerated or endured; extremely difficult to bear.
The heat was unendurably intense during the summer months.
Unendurably → It is formed from "un-" (meaning not), "endure" (from Latin "durare", meaning to last or to endure), and "-ably" (meaning in a manner that can be). The word means in a manner that cannot be endured or tolerated.
Think of 'not' ('un-') being able to 'last' ('durare') — that's why unendurably means something that cannot be tolerated.