Inexcusably means in a way that cannot be justified or forgiven because it's so clearly wrong or unacceptable. When something happens inexcusably, there's no good reason or valid excuse for it.

If a student arrives at school inexcusably late because they decided to play video games instead of leaving on time, that's different from being late because of a family emergency or a flat tire. The video game delay can't be justified. When a referee makes an inexcusably bad call in a championship game by ignoring an obvious rule, everyone recognizes that no explanation could make that decision acceptable.

The word suggests that something crosses a line beyond ordinary mistakes or understandable failures. Everyone messes up sometimes, and good excuses exist for many problems. But inexcusably describes behavior so careless, thoughtless, or deliberately wrong that no reasonable explanation exists. If a captain inexcusably abandons their team during a crucial moment, or if someone is inexcusably rude to a person who's been nothing but kind to them, we recognize that basic standards of decent behavior have been violated.

The related adjective is inexcusable, meaning something that cannot be excused: “Copying someone's homework word-for-word is inexcusable.”