To surprise or confuse someone so much that they are left speechless or unable to respond.
From 'dumb' (unable to speak) plus 'found,' possibly influenced by 'confound.' The word emerged in the 1600s to describe being so shocked you lose your ability to speak.
It's a brilliant word because it literally means 'to make someone unable to speak'—the word itself describes the physical sensation of shock: mouth open, words stuck, silence.
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