Inexcusable means something is so wrong or unacceptable that there's no good reason or explanation that could make it okay. When something is inexcusable, you can't excuse it or defend it with an explanation, no matter how hard you try.

If you accidentally knock over someone's juice at lunch, that's forgivable because accidents happen. But if you deliberately trip someone in the hallway because you're angry, that's inexcusable behavior. There's no explanation that makes it acceptable. Similarly, copying answers on a test is inexcusable because everyone knows the rules, and breaking them deliberately is wrong.

Some mistakes can be excused because they were honest errors or because circumstances made them understandable. But inexcusable actions are different: they're so clearly wrong that no excuse works. If a friend constantly breaks promises and lies about it, you might eventually say their behavior has become inexcusable.

Teachers might call it inexcusable when a student doesn't even try to complete their homework without a legitimate reason. Parents might find it inexcusable if you're cruel to a younger sibling. The word carries serious weight: it means there's a line, and someone has crossed it.