A dud is something that fails to work or turns out to be disappointing after you expected it to be good. The word originally described explosives that didn't go off: imagine a firework that just sits there instead of launching into the sky with a beautiful burst of color and light. That's a dud.

Today we use the word more broadly. A movie that everyone thought would be exciting but turns out boring is a dud. A joke that falls flat is a dud. When you're really looking forward to a new video game but it has terrible graphics and boring gameplay, you might call it a dud. A plan that seems brilliant but completely fails when you try it? Also a dud.

The word carries a sense of letdown or anticlimax. Something is a dud when it fails to deliver what it promised. A dud firework doesn't go off. A dud plan doesn't work. A dud party never gets fun. There's always this gap between what you hoped for and what actually happened.

You'll sometimes hear the phrase a real dud to emphasize just how disappointing something turned out to be.