So shocked or surprised that you are unable to speak; rendered speechless by amazement.
Compound word from 'dumb' (unable to speak) and 'struck' (past participle of strike), originally meaning 'struck into dumbness,' popularized in the 1700s.
This word perfectly captures the physical sensation of shock—the image of being literally struck or hit so hard you can't speak anymore, which is why it's so much more vivid than just saying 'surprised.'
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