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Phrase(s): Still waters run deep.
Prov. Quiet people are often very thoughtful. • Jill: I get the impression that Nathan is not very smart. He never says anything. Jane: Don’t be so sure. Still waters run deep, you know.
McGrawhill's American Idioms And Phrasal Verbs
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A quiet person may be very profound, as in Susie rarely says much, but still waters run deep. The physical observation in this term dates from ancient times, but it has been used figuratively since about 1400. Anthony Trollope amplified it in He Knew He Was Right (1869): "That's what I call still water. She runs deep enough. . . . So quiet, but so? clever."
American Heritage Idioms