To be undone means to be ruined, destroyed, or brought to a state of failure, often by a single weakness or mistake. When a general is undone by overconfidence, their pride leads to defeat. When a student's careless error undoes hours of careful work, one small mistake ruins the whole result.

The word carries a sense of tragedy: something that was built up or going well gets torn down or falls apart. In classic stories, a villain might be undone by their own greed, or a hero undone by a fatal flaw. Shakespeare often showed characters whose success or life came undone because of one critical weakness they couldn't overcome.

Undone can also simply mean not finished or not fastened. When your shoelaces come undone, they've untied themselves. If homework remains undone, it hasn't been completed yet. This everyday meaning is much less dramatic, but it shares the sense of something that was together now being apart, or something that should be complete remaining incomplete.

The verb form undo means to reverse something, like hitting “undo” after making a mistake on the computer, or to unfasten something, like when you undo the buttons on a coat.