1. a person or thing that dumbfounds; something that astonishes or confuses someone greatly
Usage: often used informally; less common than the verb 'dumbfound'

Examples

• The magician's final trick was a real dumbfounder that left the audience speechless.
• His unexpected resignation was a dumbfounder for everyone at the company.
• The plot twist in the movie was such a dumbfounder that no one saw it coming.
• That answer on the test was a complete dumbfounder—I had no idea how to respond.