“Still waters run deep” is a Latin proverb now commonly taken to mean that a placid exterior hides a passionate or subtle nature. Formerly it also carried the warning that silent people are dangerous, as in Suffolk’s comment on a fellow lord in William Shakespeare’s play Henry VI part 2:
Smooth runs the water where the brook is deep,
And in his simple show he harbors treason…
No, no, my sovereign, Gloucester is a man
Unsounded yet and full of deep deceit.
According to The Concise Oxford Dictionary of Proverbs, the first mention of the proverb appeared in classical times as “the deepest rivers flow with least sound” in a history of Alexander the Great by Quintus Rufus Curtius. The earliest use in English sources goes back to 1400. The French have a proverb: “no water is worse than quiet water.”
In 1692 Roger L’Estrange included an outline of the Abstemius version in his edition of the fables under the title of A Country-man and a River, along with the interpretation that men of few words are dangerous: “A Country-man that was to pass a River, sounded it up and down to try where it was most fordable: and upon Trial he made this Observation on’t: Where the Water ran Smooth, he found it Deepest; and on the contrary, Shallowest where it made most Noise. There’s More Danger in a Reserv’d and Silent, than in a Noisy, Babbling Enemy.”
There was a version of the story in La Fontaine’s Fables under the title “The torrent and the river.” It tells of a man trying to escape a robber. The man easily fords a turbulent stream but drowns in a smooth-flowing river, ending on the caution that, “Silent folk are dangerous.”
I have to dispute all that proverbial advice. I’m a very quiet person and I’m not at all dangerous. In spite of being sorely tempted, I haven’t drowned anybody yet.