A person or thing that dumbfounds; something surprising or shocking enough to leave you speechless.
Agent noun from 'dumbfound,' formed by adding '-er' to indicate one who or that which dumbfounds.
This word is rare but perfectly logical—if something can dumbfound you, then by English word-building rules, it should be called a dumbfounder, yet most people just say 'shocking thing' instead.
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