Indefensible means impossible to justify, excuse, or protect. When something is indefensible, no reasonable argument can make it right or acceptable.

If a student copies an entire essay from the internet and claims it as their own work, that's an indefensible action. There's no good excuse for it. The student can't reasonably argue that what they did was okay. Similarly, if someone breaks a promise for no good reason or lies to get out of trouble, those are indefensible choices.

A castle built on flat, open ground with no walls might be indefensible. Today we use the word more often to describe actions, decisions, or arguments that can't be defended with logic or ethics.

When someone does something indefensible, even their friends might say, “I can't defend what you did.” Notice how that works: defend means to protect or argue in favor of something, while indefensible means beyond any reasonable defense. If your position in an argument is indefensible, no amount of clever talking will make it stand up to careful questioning. The facts simply won't support you.