1. a person or thing that dumfounds; something that astonishes or confuses greatly
Usage: informal; often used in the phrase 'a real dumfounder'

Examples

• The magician's final trick was a real dumfounder that left the audience speechless.
• His sudden resignation was a dumfounder to everyone in the office.
• The plot twist in the movie was a complete dumfounder.
• That answer on the test was a dumfounder—I had no idea how to solve it.