Struck thus, one can’t speak (4)

I believe the answer is:

dumb

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But I can't explain it at all! Could you help me understand?

(Other definitions for dumb that I've seen before include "Lacking speech" , "Stupid (sl.)" , "Thick" , "Stupid; unable to speak" , "Struck with amazement" .)

I've seen this clue in The Hindu.

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