Unheard-of means so unusual or extreme that it's never happened before, or at least is extremely rare. When something is unheard-of, people react with surprise because they've literally never heard of such a thing happening.

If your teacher announced that everyone in class scored 100% on a difficult test, that might be unheard-of: possibly it's never happened in the school's history. When the first person ran a mile in under four minutes in 1954, many scientists thought it was unheard-of, even impossible.

The phrase captures extraordinary events that break what people thought were the limits. A blizzard in July would be unheard-of in most places. A fourth-grader writing a novel might be unheard-of in your town, though it's certainly been done before somewhere.

People sometimes exaggerate and call merely surprising things unheard-of when they're actually just uncommon. True unheard-of events are the ones that make people say, “I never imagined that could happen,” expressing something beyond ordinary surprise.