IPA: /ɪnˈɛkʃʊzəblnəs/
KK: /ɪnˈɛkʃʊzəblnəs/
The state or quality of being unable to be justified or excused; a serious fault or wrongdoing that cannot be forgiven.
The inexcusableness of his actions shocked everyone in the community.
Inexcusableness → It is formed from "in-" (meaning not), "excusable" (from Latin "excusabilis", meaning able to be excused), and "-ness" (meaning the state of being). The word refers to the state of being not able to be excused or justified.
Think of 'not' ('in-') being able to be 'excused' ('excusable') — that's why inexcusableness means the state of being unable to justify something.