Still waters run deep

Still waters run deep is a proverb of Latin origin 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 harbours treason...

No, no, my sovereign, Gloucester is a man

Unsounded yet and full of deep deceit.^[1]

According to The Concise Oxford Dictionary of Proverbs, the first mention of the proverb appeared in Classical times in the form altissima quaeque flumina minimo sono labi (the deepest rivers flow with least sound) in a history of Alexander the Great by Quintus Rufus Curtius and is there claimed as being of Bactrian origin.^[2] The earliest use in English sources goes back to 1400.^[3]

^ 3.i.53–57
^ De Rebus Gestis Alexandri Magni, VII. iv. 13
^ The Concise Oxford Dictionary of Proverbs, "STILL waters run deep", p. 307 in the 2003 e-edition

[1] 3.i.53–57

[2] De Rebus Gestis Alexandri Magni, VII. iv. 13

[3] The Concise Oxford Dictionary of Proverbs, "STILL waters run deep", p. 307 in the 2003 e-edition

[1]

[2]

[3]