still waters run deep

still waters run deep idiom
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.”

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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