Inexcusable means too bad to be forgiven or explained away. Inexcusable behaviour, an inexcusable delay, quite inexcusable.
It closes the door on explanation, which is what makes it a strong word: saying something is inexcusable is refusing to hear the reasons. It is often the second half of a sentence whose first half admits the reasons exist.
1adjectiveinexcusableToo bad to be forgiven or explained away.
“Forgetting your own daughter's birthday is inexcusable.”
inexcusable is a quality.
Latin inexcusabilis, in-, not, plus excusare, to free from blame, from causa, a cause or case.
Latin
inexcusabilis
Middle French
inexcusable
English, today
inexcusable
From Middle French inexcusable, from Latin inexcusabilis.
Etymology from Wiktionary, CC BY-SA