Shocked and unable to speak or react because something was so surprising or unexpected; astonished and confused.
From 'dumb' (unable to speak, Old English 'dumb') and 'found' (archaic past tense of 'founder' meaning to fall or fail). Combined around 1700s.
The word combines 'dumb' (mute) and 'founded' (defeated/overthrown)—your ability to speak is literally overthrown by shock, making it a perfectly descriptive image for how surprise paralyzes us.
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